General Physiology: An Outline of the Science of Life

tions from this direction, which are produced by spontaneous impulses. The essential factor in all directive reactions is, therefore, the assumption of an axial position by the cell-body, and the explanation of this is the key to an understanding of the mechanics of these phenomena. We will now study the mechanism by which the axial position is assumed in various types of free-living cells. The simplest and clearest relations are, as always, in naked protoplasmic masses, such as Amoeba and leucocytes. Let us imagine an Amoeba in a spherical form about to move, and an excitation of contraction to appear at one spot as a result of a stimulus acting unilaterally (Fig. 246, a). The excitation would be least at the portion of the surface of the sphere opposite the place of stimulation. The protoplasm there would flow out unhindered, while upon the stimulated side the strong contraction would allow no bulging. The protoplasm would, therefore, form a pseudopodium toward the unstimulated side (Fig. 246, 5). Thus an Amceba, which under conditions that are equal upon all sides extends its pseudopodia in all directions and creeps sometimes here and sometimes there, would now assume an axially differentiated form (Fig.

FIG. 246. — Scheme of axial orientation in an Amceba resulting from an excitation of contraction upon the right side. The thickness of the contour indicates the excitation. The arrows indicate the direction of the creeping. 246, c), as is the case in that variety called Amceba Umax. Under these circumstances with continual stimulation upon the same side the Amceba would necessarily creep gradually away from the source of the stimulus, as is the case in the negative chemotaxis and thermotaxis of Amceba, Myxomycetes, leucocytes, etc. Vice versa, if the spherical Amceba were to be acted upon on one side by a stimulus that produces a local excitation of expansion, the protoplasm would flow out most strongly toward that side, so that with continual stimulation the Amceba must necessarily approach the source of the stimulus. The positive chemotaxis of leucocytes, Amceba, Myxomycetes, and other naked protoplasmic masses is thus explained. If a stimulation of expansion acts upon one side of an Amoeba, and a stimulation of contraction upon the other side, the results of the two must naturally be expressed in like senses, i.e., the Amceba must creep away from the latter side and toward the former. The galvanotaxis of Amceba affords an unusually clear proof of this (Of. Fig. 232, p. 458).

The mechanism of axial orientation is less complex in those microscopic organisms that do not consist of protoplasmic masses that are constantly changing their form, but, like Bacteria and Infusoria, possess an axially differentiated body of a constant form, which moves through the water by means of special organoids of motion, flagella and cilia. The bodies of these organisms are driven through the water after the manner of a row-boat by the rhythmic stroke of the flagella or cilia. The analogy of the motion to that of a boat moved by oars is complete, and may be recognised even in details. Exactly the same means are used to turn and direct the movements of the rowed boat, and the movements of the free-swimming ciliated cell, and we can represent the peculiar behaviour of Bacteria and Infusoria in their axial orientation upon unilateral stimulation no better than by means of this simile. Among the various organisms that propel their extended bodies through the water by means of flagella or cilia, three types can be distinguished as most important, according as they move by means of a single flagellum, two flagella, or several or very many cilia, corresponding to a boat propelled by one, two, or many oars.

We will examine, first, the forms that possess - -, one flagellum, such as many Bacteria and flagellate Infusoria, and will select as representative the delicate, green, flagellate-infusorian J&6#/e.?w?, which, in summer, by means of its countless numbers, changes the water of standing pools into a deep green. The flagellum of the Flagellata is upon the anterior pole of the body and moves through the water in a screw-like path. For the sake of simplicity its motion FIG. 247. ^Scheme of the

may be considered as taking place in a single flagellum of a flageiplane. It is then seen that it oscillates about late-infusorian ceil, the straight middle position (Fig. 247, a) by means of alternate rhythmic contractions toward the right (b) and toward the left (b^) ; the swing out of the middle position (a) into one of the two extreme positions (b or bj represents the phase of contraction, the return from one of the extreme positions into the middle position the phase of expansion. The flagellum works, therefore, like an oar that is moved alternately to the right and to the left at the bow of a boat. It is evident that, while undisturbed and having equal conditions upon all sides, the infusorian body must move forward in a straight line, if the flagellum beats equally strongly toward the right and toward the left, i.e., if contraction and expansion occur with equal rapidity toward the two sides. But if a contractile stimulus acts upon the flagellate suddenly from one side, and if the long axis of the body is not already turned in the direction of the stimulus with the posterior pole toward its source, such a position is assumed by means of a few strokes of the flagellum ; for with every oblique or

transverse position of the long axis the flagellum is stimulated to contract more strongly upon the side "upon which the stimulus falls than upon the opposite side, it makes stronger strokes toward the former than toward the latter side, and the result is that the anterior part of the body is turned away from the source of the stimulus (Fig. 248). Exactly the same relations exist here as in a boat moved by a single oar. The bow of the boat also turns toward the opposite side when the boat is propelled more strongly upon one side than the other. The unequal strength of the flagellar stroke in the two directions continues, and the anterior part of the body is turned constantly more away from the source of the stimulus, until the body has placed its long axis in the direction of the incident stimulus (Fig. 248, d). Then both sides of the flagellum become equally stimulated and the protist swims in a straight line, so long as the stimulus continues. Thus, negative chemotaxis, phototaxis, etc., appear in uniflagellated

Fia. 248.— Scheme of axial orientation of a flagellate-infusorian cell, resulting from an excitation of contraction upon the right side. The side toward which the concavity of the flagellum is directed is the stimulated side. The arrows indicate the direction of movement. Bacteria and Flagellata as a necessary result of a unilateral excitation of contraction in the flagellum. It is now very easy to imagine the relations as regards axial direction in forms possessing two flagella, such as the flagellate infusorian Polytoma (Fig. 249). If two flagella are present upon the anterior end of the cell, the latter corresponds to a boat that is moved by two oars at the bow. If the strokes of the two oars are equal, the boat moves in a straight line. It is the same with the flagellated cell. If one oar moves more strongly, the bow of the boat is turned toward the opposite side. The same will occur with the cell possessing two flagella, when a contractile stimulus acts upon one side, causing one flagellum to beat more strongly than the other (Fig. 249, a, b, c). The anterior end of the cell must then be turned away from the source of the stimulus, until the long axis is turned in the direction of the latter, with the posterior end nearer it. In this direction the two flagella are equally stimulated (Fig. 249, d), and as a result the

flagellate swims straight away from the source of the stimulus. In this way negative chemotaxis, etc., occur in the forms possessing two flagella, by reason of unilateral excitation of contraction. The same thing that occurs in Polytoma and other forms as the result of the activity of two flagella, occurs in ciliate Inficsoria by means of the beat of numerous cilia. The movements of Paramcecium are analogous to the movements of a long boat possessing many oars. If all the oars upon the two sides move with exactly equal force, the boat moves straight forward ; if the stroke of the oars is stronger upon one side than upon the other, the boat turns toward the opposite side. The same is true of the ciliary movement in Param&cium. If the cilia beat with equal strength upon the two sides, the infusorian swims forward in a straight line ; if, however, a contractile stimulus acts upon one side, so that the cilia upon that side are made to beat more strongly than upon the other (Fig. 250, a), the anterior end of the body must turn away

FIG. 249. — Scheme of axial orientation of a biflagellated infusorian cell, resulting from an excitation of contraction upon the right side. The greater concavity of one flagellum indicates the stronger excitation. The arrows indicate the direction of movement. from the source of the stimulus until its long axis is placed in the direction of the latter. The cilia then become stimulated equally upon corresponding points of the two sides of the body, and the cell swims forward in a straight direction away from the source of the stimulus. In this way negative chemotaxis, barotaxis, thermotaxis, and phototaxis occur in ciliate Infusoria from unilateral excitation of contraction.

The mechanism of axial orientation in the positively tactic movements of ciliated cells is likewise simple. Orientation in these cases can be called forth by a unilateral excitation of expansion. If such a stimulus acts upon one side, the expansion of the ciliary stroke, i.e., the return of the flagellum or the cilium to the resting-position, will then take place more energetically upon this side of the body than upon the opposite side. The result will be the reverse of that when the contraction is more energetic, i.e., the anterior end of the body will be turned toward the side of the incident stimulus, until the long axis is

placed in the direction of the stimulus. It depends then upon the relative extent of the phases of contraction and expansion of the cilia, whether the motor effect will be so directed that the cell in taking the axial position will move toward the source of the stimulus or away from it. The axial position must always be taken, however, whether the cell moves by means of one cilium, by two, or by many (Fig. 251). If, finally, an excitation of contraction occurs upon one side of the infusorian cell and an excitation of expansion upon the other

FIG. 250. — Scheme of axial orientation of a ciliate infusorian, resulting from an excitation of contraction upon the right side. The greater concavity of the cilia backwards indicates the stronger excitation. The arrows indicate the direction of the movement and are placed at the anterior pole of the body side, it is evident that as regards the axial position of the body, the two must act in the same sense, i.e., so that the anterior end is directed toward the side of the expansory stimulus. Whether in this position a movement in one or the other direction or a standstill takes place, depends wholly upon the extent and

FIG. 251. — Scheme of axial orientation of a ciliate infusorian. resulting from an excitation of ex- -pansion upon the right side. The arrows are at the anterior pole of the body and indicate the direction of movement. The greater concavity of the cilia forward indicates the excitation. direction of the motor effect exerted by the ciliary stroke at the two ends of the body. That all these three possibilities are realised, is shown most beautifully by galvanotaxis, in which, according to the intensity of the galvanic current, swimming forward or backward, or standstill, can be obtained.

From the foregoing considerations the mechanism of axial orientation resulting from a depression of contraction or expansion upon one side of the body may be at once understood. In such a case there exists a difference in the activity of the organoids of movement on the two sides of the body, and, according to known principles, this must cause a rotation upon the axis until the difference is balanced. If we add to our first scheme the various axial positions that a cell -body, differentiated as regards its poles either temporarily or permanently, may assume by reason of an excitation or depression of contraction or expansion at one pole, we obtain the following cases, in which the points of the arrows indicate the position of the anterior end of the body :

fcf Expressed in words, this means : the anterior pole of the body turns away from the source of the stimulus with excitation of contraction or depression of expansion upon one side ; • and toward the source of the stimulus with depression of contraction or excitation of expansion upon one side. Whether the cell in axial orientation moves forward or backward, or stands still, depends in. a given case upon the relation, as regards intensity, of the phase of contraction to that of expansion in the whole cell.

Thus the phenomena of positive and negative chemotaxis, barotaxis, thermotaxis, phototaxis and galvanotaxis which are so highly interesting and important in all organic life, follow with mechanical necessity as the simple results of differences in biotonus, which are produced by the action of stimuli at two different poles of the free-living cell. We recognised with Pfliiger as the essential part of metabolism the continual construction and destruction of certain proteidlike compounds of very labile constitution. Although at present any chemical characterisation of these is unsatisfactory, we termed them, in brief, biogens because of their great significance for life, and we defined the vital process in the simple schematic form : the sum of all the processes that are associated with the construction and destruction of biogens.

The non-living matter that enters into the living substance from outside is manufactured continually into living matter by complex transformations in the living substance ; it also dies continually and is excreted as non-living matter. Thus, life consists of an eternal process of becoming alive and dying, which go on in all living substance at every moment side by side and uninterruptedly. The sum of all processes associated with the construction of living substance forms the phase of assimilation, the sum of all processes

associated with the destruction of living substance forms the phase of dissimilation. Assimilation and dissimilation are the basis of all life. Their relation to one another, which was termed biotonus, controls vital phenonema. From the beginning of development to death biotonus is constantly changing by reason of individual members of the series of assimilation or dissimilation assuming different values, and thus the details of the vital phenomena likewise change. Biotonus likewise changes if stimuli act upon the living substance, and, accordingly, vital phenomena change under the influence of stimuli. Thus, vital phenomena are determined by the individual links in the long chain of metabolism, which together form the vital process.

After having recognised metabolism as the elementary vital process, our present task is to derive mechanically from metabolism the vital phenomena, which must be regarded as the expression of the vital process. It has been seen that all living substance that now inhabits the earth's surface possesses the form of cells : hence the cell is the proper seat of the vital process. In the cell the general vital phenomena are found in their elementary form. If, therefore, their mechanical analysis is not to stop half-way, the cell must be made the object of investigation. Before we can expect to derive the various vital phenomena of the cell mechanically from its metabolism, the question must be answered, how the metabolism of living substance, which thus far we have schematically conceived to take place in a uniform substratum, goes on in the cell which possesses the characteristic differentiations of its contents. Although with our very slight knowledge of the individual chemical processes in the cell, we are quite unable to picture in detail its more delicate metabolic mechanism, the investigations of the last decade have afforded sufficient material to give an idea of its general metabolic relations. These investigations have revealed a large number of facts, which allow a definite conclusion to be drawn regarding the much-discussed significance of the two essential cell-constituents, the nucleus and the protoplasm, as well as regarding the nature of their relation to one another.

The classical researches of the earlier investigators of protoplasm, among whom may be named Dujardin and Max Schultze, were directed toward establishing the protoplasm as the bearer of all vital activities. In the older doctrine of the cell all perceptible vital phenomena were regarded as taking place in the protoplasm, and no one knew what to do with the nucleus ; hence the latter was considered unessential and received little attention. It is psychologically interesting and a characteristic phenomenon in the history of human thought that the knowledge of the truth swings to either side of the middle point before it comes to a standstill at the latter. An extreme view, which in the course of time proves to be untenable, causes a swing to the opposite extreme, and the true medium is found only gradually by means of a healthy reaction. Thus it was with the cell-doctrine.

After it was found that the nucleus undergoes profound changes, especially in the reproduction of the cell by division and in the fertilisation of the ovum, while the protoplasm remains apparently quiet, the original idea of the dominant role of the protoplasm changed to the opposite one of control by the nucleus. The latter regarded the nucleus as the essential bearer of the cell-life, and the protoplasm as performing merely an accessory function. What in the earlier cell -doctrine was ascribed exclusively to the protoplasm, in the later one was ascribed exclusively to the nucleus. During the last few years a healthy reaction against this swing to the other extreme is beginning to make itself felt.

It is not possible to examine all the individual facts, relative to the function of the nucleus and the protoplasm, that have recently been brought together. It will suffice to indicate some of the more important observations and experiments that have led to important deductions. The idea that the nucleus plays a dominant role in the cell has obtained at the present time wide acceptance and has been expressed in various forms. One prominent view is defended by eminent investigators, such as Weismann, Hertwig, Boveri and others, and takes special account of the remarkably complex and regular changes that the newer morphology has demonstrated in the nucleus in connection with the phenomena of fertilisation and division of the ovum. The essence of this view is that the nucleus is the bearer of certain substances called hereditary substances, that hereditary transmission takes place by means of a transference of these substances to the descendants, and that the protoplasm contains no substances necessary to heredity.

The fact that in the fertilisation of the ovum by the spermatozoon only a very small quantity of protoplasm is transferred by the latter to the descendants, since the spermatozoon consists in by far the greater part of nuclear substance, induced biologists to neglect completely this small quantity of protoplasm, and to ascribe the transference of the paternal characteristics to the descendants exclusively to the nucleus of the spermatozoon. This assumption appeared the more probable because the small mass of protoplasm in the spermatozoon, which is contained chiefly in the flagellum,

cannot be distinguished from the protoplasm of the ovum after it has appeared in the latter, while the characteristic and profound changes caused by fertilisation appear in the nucleus alone. To the more critical minds, the weakness of the arguments, upon which rested the theory of the dominance of the nucleus in heredity, was painfully evident, and they sought after unequivocal proofs of the theory. The fundamental fact, which Nussbaum ('84, '86) established in Infusoria, that non-nucleated pieces of a cell after some time invariably die, while nucleated pieces are regenerated into complete cells and continue to reproduce by cell-division, was experimentally confirmed in other Infusoria by Gruber ('85), and brought forward as a direct proof of the dominance of the nucleus. Gruber ('86, 1) says : " By a purely empirical method we are here placed before the undeniable fact that the nucleus is the most important, the species-maintaining, constituent of the cell, and that to it is rightly ascribed the highest significance in the processes of fertilisation and hereditary transmission." But Gruber forgets that, in order that the nucleus may be established as the sole species-maintaining constituent of the cell, the reverse experiment must also be made, namely, the investigation of the nucleus without the protoplasm. If the nucleus then continues to live, if it regenerates a new protoplasmic body and forms a complete individual, the experiment would be, in fact, undeniable proof of the all-important significance of the nucleus. But if it perishes without regeneration, like the protoplasm deprived of its nucleus, no reason then exists for ascribing more to the nucleus than to the protoplasm ; with equal right the protoplasm could then be spoken of as the species-maintaining constituent of the cell. Such an experiment has been performed and shows that the nucleus deprived of its protoplasm perishes like non-nucleated protoplasm. In the large radiolarian Thalassicolla (Fig. 171, p. 380) the nucleus, which is visible to the naked eye, can by a skilful operation with delicate instruments be removed uninjured from the protoplasm of the central capsule, and be observed isolated.

The result is that, even when it is protected from all injury, after some time it invariably dies without a trace of regenerative phenomena being seen.1 The same may be observed in Infusoria. Such a result breaks the force of Gruber's argument. Another experiment, which is claimed to support the theory of the dominance of the nucleus, was performed by Boveri ('89) upon the eggs of the sea-urchin. In connection with the fact observed by the brothers Hertwig ('87), that non-nucleated pieces of protoplasm of the ova of sea-urchins are capable of being fertilised by spermatozoa, Boveri found that these fertilised pieces develop into dwarf larval forms, which, apart from their small size, are

wholly like normal larvae. Boveri employed this fact in experiments upon the cross-fertilisation of non-nucleated pieces of ova of one species of sea-urchin with spermatozoa of another species, especially non-nucleated pieces of ova of Sphcerechinus granularis with spermatozoa of Echinus microtuberculatus. He shook a number of eggs of Sphcerechinus in a test-tube, by which treatment non-nucleated pieces of protoplasm were broken off, and fertilised the shaken liquid with sperm of Echinus. It was not possible to fertilise isolated non-nucleated pieces, since cross-fertilisation between the two forms takes place relatively very seldom. Among the larvae obtained by fertilisation were the following :

a. Bastards, such as were always obtained by the crossing of the two species. b. Dwarf bastards, obtained by the fertilisation of nucleated pieces. c. Dwarfs possessing genuine Echinus characters, obtained by the fertilisation of non-nucleated pieces. According to Boveri, the occurrence of the last-mentioned larval forms is a direct proof of the theory of nuclear dominance ; for, since from one species of sea-urchin only non-nucleated protoplasm from the ovum was transmitted, while from the other the nucleus of the spermatozoon, the result, namely, larvae of the paternal form, proves that the nucleus alone can be the bearer of the hereditary substances. Critical examination, however, shows that this experiment, which thus far has been considered by many as the strongest support of the theory of dominance, appears as such inadequate in more than one respect. In the first place, the derivation of dwarf larvae of the type of the paternal species, can be doubted. Since the fertilisation of non-nucleated pieces of ova of one species with spermatozoa of the other species was not carried on isolated,. it is very questionable whether the larvae in question were really derived from such a fertilisation. It is conceivable that larvae pre-eminently of the paternal form can develop from the fertilisation of nucleated pieces of ova or whole ova of one form with spermatozoa of the other ; we see that very frequently the characteristics of either the father or the mother are transmitted pre-eminently to the offspring. But the yarious larval forms that Boveri obtained possess in the developmental stage in question so few distinguishing marks, that from their presence conclusions ought not to be drawn with certainty regarding the one-sided derivation. Yet, even if the explanation that Boveri gives of the derivation of the larvae is to be accepted, the experiment is far from conclusive. It will be conclusive only when the nucleus alone, not the whole spermatozoon, unites with the non-nucleated protoplasm of the ovum.

If in this case larvae of the character of the father appear, we shall be obliged to acknowledge that the nucleus alone is the bearer of hereditary characters. Since, however, the spermatozoon is a complete cell possessing nucleus and protoplasm, nothing in Boveri's experiment has proved that the protoplasm also does not take part in hereditary transmission. The fact that pre-eminently or exclusively paternal characters are observed in the larvas ought hardly to cause surprise, since upon the paternal side a whole cell enters into the fertilisation, but upon the maternal side only a bit of protoplasm, which, as is well known, is destined to die because of the loss of its nucleus, and is no longer able to preserve its characteristic features, and consequently not able to transmit them. Hence, in the light of the fundamental fact of the infallible death of non-nucleated protoplasmic masses, Boveri's view, that in his experiment maternal characteristics would necessarily have been transmitted also, if the protoplasm were to take part in heredity like the nucleus, appears unsupported. After all the above we cannot help considering Boveri's experiment wholly indifferent as regards the decision of the question whether the features of the cell that determine its character are contained in the nucleus alone.

Another form of the supremacy theory is expressed in the view of Eimer ('88), Hofer ('89-90), and others, that the nucleus controls the vital phenomena of the cell, especially the movements of the protoplasm, after the manner of a central nervous organ. Eimer supports his view by various morphological observations, not wholly undisputed, upon the ending of nerve-fibres in the nuclei and even in the nucleoli of cells. But, even if these should really be confirmed, there would be no ground in them for ascribing to the nucleus alone the regulation of the movements of the protoplasm. Hofer believes that from experiments on Amoeba he can draw the conclusion that "the nucleus is a regulatingcentre for movement." He cut the bodies of large Amoebae into nucleated and non-nucleated parts. While the nucleated parts continued to behave exactly like complete Amoebae, the non-nucleated pieces showed normal behaviour for 15 — 20 minutes only. Then the movements became irregular, the formation of pseudopodia taking place abnormally, and finally wholly ceased. From this Hofer concludes that the protoplasm possesses the power of movement, but that the nucleus is a centre which regulates the movements. That this view cannot be held follows from the striking experiments of Balbiani ('88), who observed that under favourable conditions non-nucleated pieces of Infusoria continue to live for many days with completely unchanged movements. Finally, exhaustive experiments upon various Rhizopoda and In- fusoria} especially upon ciliate Infusoria that perform very complex and characteristic movements, have been directed particularly toward this question, whether the nucleus ought to be regarded as 1 Cf. Verworn ('89, 1).

the centre of motion in the same sense as the centres of the central nervous system in higher animals are regarded. We can present the result of these best by means of a vivisection-experiment upon Lacrymaria. Lacrymaria olor belongs to the holotrichous Giliata and is distinguished by its very characteristic movements ; from these it proves to be an exceptionally favourable object for experiment upon the influence of the nucleus upon movement. In the condition of moderate contraction it is flask-shaped and presents a trunk, neck, and head (Fig. 252, a and V). When undisturbed, it is in restless motion, every portion of the cell-body taking part in its peculiar activity. The trunk undergoes constant changes of form of a peristaltic character. At times the neck extends into an extremely long and slender thread, the anterior end of which lengthens, shortens, bends about and gropes here and there between the particles of mud (Fig. 252, a\ and at times it suddenly draws together like a stretched rubber cord, soon to begin its play anew. The head, provided with long oral cilia, gropes about in all directions upon objects in the water, the cilia seeming to run over them like little feet. In this way the whole protist twitches constantly forward and backward by the alternate direction of the strokes of the cilia, so that it moves very little from one place and is engaged chiefly in searching about with its long neck and head with restless eagerness. If it be stimulated, it suddenly contracts and swims in the condition of moderate contraction some distance backward, then takes a forward direction and whirls forward through the water at a furious rate, constantly turning about its axis.