On Growth and Form

+ They represent the general theorem of which particular cases are found, for instance, in the asymmetry of the ferments (or enzymes) which act upon asymmetrical bodies, the one fitting the other, according to Emil Fischer’s wellknown phrase, as lock and key. Cf. his Bedeutung der Stereochemie fiir die Physiologie, Z. f. physiol. Chemie, v, p. 60, 1899, and various papers in the Ber. d. d. chem. Ges. from 1894. crux of the difficulty; for they at once bid us enquire whether in nature, external to and antecedent to life, there be not some asymmetry to which we may refer the further propagation or “begetting’” of the new asymmetries: or whether in default thereof, we be rigorously confined to the conclusion, from which Japp “saw no escape,” that “at the moment when life first arose, a directive force came into play,—a force of precisely the same character as that which enables the intelligent operator, by the exercise of his will, to select one crystallised enantiomorph and reject its asymmetric opposite *.”

Observe that it is only the first beginnings of chemical asymmetry that we need to discover; for when asymmetry is once manifested, it is not disputed that it will continue “to beget asymmetry.” A plausible suggestion is now at hand, which if it be confirmed and extended will supply or at least sufficiently illustrate the kind of explanation which is required f. We know in the first place that in cases where ordinary nonpolarised light acts upon a chemical substance, the amount of chemical action is proportionate to the amount of light absorbed. We know in the second place, in certain cases, that hght circularly polarised is absorbed in different amounts by the right-handed or left-handed varieties, as the case may be, of an asymmetric substance. And thirdly, we know that a portion of the lght which comes to us from the sun is already plane-polarised light, which becomes in part circularly polarised, by reflection (according to Jamin) at the surface of the sea, and then rotated in a particular direction under the influence of terrestrial magnetism. We only require to be assured that the relation between absorption of light and chemical activity will continue to hold good in the case of circularly polarised light; that is to say

* In accordance with Emil Fischer’s conception of “asymmetric synthesis,” it is now held to be more likely that the process is synthetic than analytic: more likely, that is to say, that the plant builds up from the first one asymmetric body to the exclusion of the other, than that it “selects” or “picks out” (as Japp supposed) the right-handed or the left-handed molecules from an original, optically inactive, mixture of the two; cf. A. McKenzie, Studies in Asymmetric Synthesis, Journ. Chem. Soc. (Trans.), LXxxv, p. 1249, 1904.

+ See for a fuller discussion, Hans Przibram, Vitalitdt, 1913, Kap. iv, Stoffwechsel (Assimilation und Katalyse). that the formation of some new substance or other, under the influence of light so polarised, will proceed asymmetrically in consonance with the asymmetry of the light itself; or conversely, that the asymmetrically polarised light will tend to more rapid decomposition of those molecules by which it is chiefly absorbed. This latter proof is now said to be furnished by Byk*, who asserts that certain tartrates become unsymmetrical under the continued influence of the asymmetric rays. Here then we séem to have an example, of a particular kind and in a particular instance, an example limited but yet crucial (if confirmed), of an asymmetric force, non-vital in its origin, which might conceivably be the starting-point of that asymmetry which is characteristic of so many organic products.

The mysteries of organic chemistry are great, and the differences between its processes or reactions as they are carried out in the organism and in the laboratory are many}. The actions, catalytic and other, which go on in the living cell are of extraordinary complexity. But the contention that they are different in kind from what we term ordinary chemical operations, or that in the production of single asymmetric compounds there is actually to be witnessed, as Pasteur maintained, a “prerogative of life,” would seem to be no longer safely tenable. And furthermore, it behoves us to remember that, even though failure continued to attend all artificial attempts to originate the asymmetric or optically active compounds which organic nature produces in abundance, this would only prove that a certain physical force, or mode of physical action, is at work among living things though unknown elsewhere. It is a mode of action which we can easily imagine, though the actual mechanism we cannot set agoing when we please. And it follows that such a difference between living matter and dead would carry us but a little way, for it would still . be confined strictly to the physical or mechanical plane.

Our historic interest in the whole question is increased by the * Byk, A., Zur Frage der Spaltbarkeit von Razemverbindungen durch Zirkularpolarisiertes Licht, ein Beitrag zur primaren Entstehung optisch-activer Substanzen, Zeitsch. f. physikal. Chemie, xurx, p. 641, 1904. It must be admitted that further positive evidence on these lines is still awanting. + Cf. (int. al.) Emil Fischer, Untersuchungen iiber Aminosduren, Proteine, ete. Berlin, 1906.

fact, or the great probability, that “the tenacity with which Pasteur fought against the doctrine of spontaneous generation was not unconnected with his belief that chemical compounds of onesided symmetry could not arise save under the influence of life *.” But the question whether spontaneous generation be a fact or not does not depend upon theoretical considerations; our negative response is based, and is so far soundly based, on repeated failures to demonstrate its occurrence. Many a great law of physical science, not excepting gravitation itself, has no higher claim on our acceptance.

Let us return then, after this digression, to the general subject of the forms assumed by certain chemical bodies when deposited or precipitated within the organism, and to the question of how far these forms may be artificially imitated or theoretically explained. Mr George Rainey, of St Bartholomew’s Hospital (to whom we have already referred), and Professor P. Harting, of Utrecht, were the first to deal with this specific problem. Mr Rainey published, between 1857 and 1861, a series of valuable and thoughtful papers to shew that shell and bone and certain other organic structures were formed “by a process of molecular coalescence; demonstrable in certain artificially-formed products fT.” Professor Harting, after thirty vears of experimental work,

ublished in 1872 a paper, which has become classical, entitled ib pap Recherches de Morphologie Synthétique, sur la production artificielle de quelques formations calcaires organiques; his aim was to pave the way for a “morphologie synthétique,’ as Wohler had laid the foundations of a “‘chimie synthétique,” by his classical discovery forty years before. Mr Charles Hatchett, Chemical Experiments on Zoophytes; with some observations on the component parts of Membrane, Phil. Trans. 1800, pp. 327-402.

Rainey, and Harting used similar methods, and these were such as many other workers have continued to employ,—partly with the direct object of explaining the genesis of organic forms and partly as an integral part of what is now known as Colloid Chemistry. The whole gist of the method was to bring some soluble salt of lime, such as the chloride or nitrate, into solution within a colloid medium, such as gum, gelatine or albumin; and then to precipitate it out in the form of some insoluble compound, such as the carbonate or oxalate. Harting found that, when he added a little sodium or potassium carbonate to a concentrated solution of calcium chloride in albumin, he got at first a gelatinous mass, or “colloid precipitate”: which slowly transformed by the

white of egg. (After Harting.) “nucleus,” and _ striated, iridescent border. (After Harting.) appearance of tiny microscopic particles, at first motionless, but afterwards as they grew larger shewing the typical Brownian movement. So far, very much the same phenomena were witnessed whether the solution were albuminous or not, and similar appearances indeed had been witnessed and recorded by Gustav Rose, so far back as 1837*; but in the later stages the presence of albuminoid matter made a great difference. Now, after a few days, the calcium carbonate was seen to be deposited in the form of large rounded concretions, with a more or less distinct central nucleus, and with a surrounding structure at once radiate and

* Cf. Quincke, Ueber unsichtbare Fliissigkeitsschichten, Ann. der Physik, 1902. concentric; the presence of concentric zones or lamellae, alterround “calcospherites’’ shewed a tendency to aggregate together t in layers, and then to assume polyhedral, or often regularly hexagonal, outlines. In this latter condition they closely resemble Fig. 198, A. Section of shell of Mya; B. Section of hingetooth of do. (After Carperiter.) the early stages of calcification in a molluscan (Fig. 198), or still more in a crustacean shell*; while in their isolated condition

* See for instance other excellent illustrations in Carpenter’s article “Shell,” in Todd’s Cyclopedia, vol. tv. pp. 556-571, 1847-49. According to Carpenter, the shells of the mollusca (and also of the crustacea) are “essentially composed of cells, consolidated by a deposit of carbonate of lime in their interior.” That is to say, Carpenter supposed that the spherulites, or calcospherites of Harting, were, to begin with, just so many living protoplasmic cells. Soon afterwards .however,

they very closely resemble the little calcareous bodies in the tissues of a trematode or a cestode worm, or in the oesophageal glands of an earthworm*. When the albumin was somewhat scanty, or when it was mixed with gelatine, and especially when a little phosphate of lime was Fig. 199. Large irregular calcareous concretions, or spicules, deposited in a piece of dead cartilage, in presence of calcium phosphate. (After Harting.) Huxley pointed out that the mode of formation. while at first sight ‘irresistibly suggesting a cellular structure,...is in reality nothing of the kind,” but *‘is simply the result of the concretionary manner in which the calcareous matter is deposited ” ; ibid. art. ““Tegumentary Organs,” vol. v, p. 487, 1859. . Quekett (Lectures on Histology, vol. u, p. 393, 1854, and Q. J. M. S. x1, pp. 95-104, 1863) supported Carpenter; but Williamson (Histological Features in the Shells of the Crustacea, Q. J. M. S. vi, pp. 35-47, 1860) amply confirmed Huxley’s view, which in the end Carpenter himself adopted (The Microscope, 1862, p. 604). A like controversy arose later in regard to corals. Mrs Gordon (M. M. Ogilvie) asserted that the coral was built up “of successive layers of calcified cells, which hang together at first by their cell-walls, and ultimately, as crystalline changes continue, form the individual laminae of the skeletal structures” (Phil. Trans. cLXxxxvit, p. 102, 1896): whereas v. Koch had figured the coral as formed out of a mass of “ Kalkconcremente”’ or “erystalline spheroids,” laid down outside the ectoderm, and precisely similar both in their early rounded and later polygonal stages (though von Koch was not aware of the fact) to the calecospherites of Harting (Entw. d. Kalkskelettes von Asteroides, Mitth. Zool. St. Neapel, 111, pp. 284-290, pl. xx, 1882). Lastly Duerden shewed that external to, and apparently secreted by the ectoderm lies a homogeneous organic matrix or membrane, “in which the minute calcareous crystals forming the skeleton are laid down” (The Coral Siderastraea radians, ete., Carnegie Inst. Washington, 1904, p. 34). Cf. also M. M. Ogilvie-Gordon, Q. J. M.S. xurx, p- 203, 1905, ete. * Cf. Claparéde, Z. f. w. Z. x1x, p. 604, 1869.

added to the mixture, the spheroidal globules tended to become rough, by an outgrowth of spinous or digitiform projections; and in some cases, but not without the presence of the phosphate, the result was an irregularly shaped knobby spicule, precisely similar to those which are characteristic of the Alcyonaria*. The rough spicules of the Aleyonaria are extraordinarily variable in shape and size, as, looking at them from the chemist’s or the physicist’s point of view, we should expect them to be. Partly upon the form of these spicules, and partly on the general form or mode of branching of the entire colony of

Fig. 200. Additional illustrations of Alcyonarian spicules: Hunicea. (After Studer.) polypes, a vast number of separate “species” have been based by systematic zoologists. But it is now admitted that even in specimens of a single species, from one and the same locality, the spicules may vary immensely in shape and size: and Professor Hickson declares (in a paper published while these sheets are passing through the press) that after many years of laborious work in striving to determine species of these animal colonies, he feels “‘ quite convinced that we have been engaged in a more or less fruitless task}.

The formation of a tooth has very lately been shown to be a phenomenon of the same order. That is to say, * calcification in both dentine and enamel is in great part a physical phenomenon; the actual deposit in both tissues occurs in the form of calcospherites, and the process in mammalian tissue is identical in every point with the same process occurring in lower organisms*.” The ossification of bone, we may be sure, is in the same sense and to the same extent a physical phenomenon.

The typical structure of a calcospherite is no other than that of a pearl, nor does it differ essentially from that of the otolith of a molluse or of a bony fish. (The otoliths, by the way, of the elasmobranch fishes, like those of reptiles and birds, are not developed after this fashion, but are true crystals of calc-spar.) Throughout these phenomena, the effect of surface-tension is manifest. It is by surface-tension that ultra-microscopic particles are brought together in the first floccular precipitate or coagulum ;

solution. (After Harting.) spherites. (After Harting.) by the same agency, the coarser particles are in turn agglutinated into visible lumps; and the form of the calcospherites, whether it be that of the solitary spheres or that assumed in various stages of aggregation (e.g. Fig. 202) f, is likewise due to the same agency. From the point of view of colloid chemistry the whole phenomenon is very important and significant; and not the least significant part is this tendency of the solidified deposits to assume the form of “spherulites,” and other rounded contours. In the phraseology of that science, we are dealing with a two-phase system, which finally consists of solid particles in suspension in a liquid (the former being styled the disperse phase, the latter the

+ The artificial concretion represented in Fig. 202 is identical in appearance with the concretions found in the kidney of Nautilus, as figured by Willey (Zoological Results, p. \xxvi, Fig. 2, 1902). dispersion medium). In accordance with a rule first recognised by Ostwald*, when a substance begins to separate out from a solution, so making its appearance as a new phase, it always makes its appearance first as a liquid}. Here is a case in point. The minute quantities of material, on their way from a state of solution to a state of “suspension,” pass through a lquid*to a solid form; and their temporary sojourn in the former leaves its impress in the rounded contours which surface-tension brought about while the little aggregate was still labile or fluid: while coincidently with this surface-tension effect upon the surface, crystallisation tended to take place throughout the little liquid

mass, or in such portion of it as had not yet consolidated and crystallised. Where we have simple aggregates of two or three calcospherites, the resulting figure is precisely that of so many contiguous soapbubbles. In other cases, composite forms result which are not so easily explained, but which, if we could only account for them, would be of very great interest to the biologist. For instance, when smaller calcospheres seem, as it were, to invade the substance of a larger one, we get curious conformations which in the closest possible way resemble the outlines of certain of the Diatoms (Fig. 203). Another very curious formation, which Harting calls a “conostat,” is of frequent occurrence, and in it we see at least a suggestion of analogy with the configuration which, in a protoplasmic structure, we have spoken of as a “collar-cell.” The

+ This rule, undreamed of by Errera, supports and justifies the cardinal assumption (of which we have had so much to say in discussing the forms of cells and tissues) that the incipient cell-wall behaves as, and indeed actually is, a liquid film (cf. p. 306). conostats, which are formed in the surface layer of the solution, consist of a portion of a spheroidal calcospherite, whose upper part is continued into a thin spheroidal collar, of somewhat larger radius than the solid sphere; but the precise manner in which the collar is formed, possibly around a bubble of gas, possibly about a vortex-like diffusion-current* is not obvious.

Among these various phenomena, the concentric striation observed in the calcospherite has acquired a special interest and importance. It is part of a phenomenon now widely known, and recognised as an important factor in colloid chemistry, under the name of “Liesegang’s Ringst.” If we dissolve, for instance, a little bickromate of potash in gelatine, pour it on to a glass plate, and after it is set place upon it a drop of silver nitrate solution, there appears in the course of a few hours the phenomenon of Liesegang’s rings. At first the silver forms a central patch of abundant reddish brown chromate precipitate; but around this, as the silver nitrate diffuses slowly through the gelatine, the precipitate no longer comes down in a continuous, uniform laver, but forms a series of zones, beautifully regular, which alternate with clear interspaces of jelly, and which stand farther and farther apart, in logarithmic ratio, as they recede from the centre. For a discussion of the ravson d’étre of

+ Cf. Harting, op. cit., pp. 22, 50: ‘‘J’avais cru d’abord que ces couches concentriques étaient produites par l’alternance de la chaleur ou de la lumieére, pendant le jour et la nuit. Mais lexpérience, expressément instituée pour examiner cette question, y a répondu négativement.” { Liesegang, R. E., Ueber die Schichtungen bei Diffusionen, Leipzig, 1907, and other earlier papers. this phenomenon, still somewhat problematic, the student must» consult the text-books of physical and colloid chemistry*.

But, speaking very generally, we may say the appearance of Liesegang’ s rings is but a particular and striking case of a more general phenomenon, namely es influence on crystallisation of the presence of foreign bodies or “impurities,” represented in this case by the ° eel or colloid matrix}. Faraday shewed long ago that to the presence of slight impurities might be ascribed the banded structure of ice, of banded quartz or agate, onyx, etc. ; and Quincke and Tomlinson have added to our scanty knowledge of the same phenomenon tf.

Besides the tendency to rhythmic action, as manifested in Liesegang’s rings, the association of colloid matter with a crystalloid in solution may lead to other well-marked effects. These, according to Professor J. H. Bowman§, may be grouped somewhat as follows: (1) total prevention of crystallisation ; (2) suppression of certain of the lines of crystalline growth; (3) extension of the crystal to abnormal proportions, with a tendency for it to become a compound crystal; (4) a curving or gyrating of the crystal or its parts.

For instance, it would seem that, if the supply of:material to the growing crystal be not forthcoming in sufficient quantity (as may well happen in a colloid medium, for lack of convectioncurrents), then growth will follow only the strongest lines of crystallising force, and will be suppressed or partially suppressed along other axes. The crystal will have a tendency to become filiform, or “fibrous”; and the raphides of our plant-cells are a casein point. Again, the long slender crystal so formed, pushing its way into new material, may initiate a new centre of crystallisation: we get the phenomenon known as a “relay,” along the

principal lines of force, and sometimes along subordinate axes as well. This phenomenon is illustrated in the accompanying figure of crystallisation -in a colloid medium of common salt; and it may possibly be that we have here an explanation, or the Hexactinellid sponges. Lastly, when the crystallising force is nearly equalled by the resistance of the viscous medium, the crystal takes the line of least resistance, with’ very various results. One of these results would seem to be a gyratory course, giving to Fig. 207. Wheel-like crystals in a the crystal a curious wheel-hke SOI. (Ae aes shape, as in Fig. 207; and other results are the feathery, fern-like

or arborescent shapes so frequently seen in microscopic crystallisation. To return to Liesegang’s rings, the typical appearance of concentric rings upon a gelatinous plate may be modified in various experimental ways. For instance, our gelatinous medium may be placed in a capillary tube immersed in a solution of the precipitating salt, and in this case we shall obtain a vertical succession of bands or zones regularly interspaced : the result being very closely comparable to the banded pigmentation which we see in the hair of a rabbit or a rat. In the ordinary plate preparation, the free surface of the gelatine is under different conditions to the lower layers and especially to the lowest layer in contact with the glass; and therefore it often happens that we obtain a double series of rings, one deep and the other superficial, which by occasional blending or interlacing, may produce a netted. pattern. In some cases, as when only the inner surface of our capillary tube is covered with a layer of gelatine, there is a tendency for the deposit to take place in a continuous spiral line, rather than in concentric and separate zones. By such means, according to Kiister* various forms of annular, spiral and reticulated thickenings in the vascular tissue of plants may be closely imitated; and he and certain other writers have of late been inclined to carry the same chemico-physical phenomenon a very long way, in the explanation of various banded, striped, and other rhythmically successional types of structure or pigmentation. For example, the striped pigmentation of the leaves in many plants (such as Eulalia japonica), the striped or clouded colourmg of many feathers or of a cat’s skin, the patterns of many fishes, such for instance as the brightly coloured tropical Chaetodonts and the like, are all regarded by him as so many instances of “ diffusion-figures” closely related to the typical Liesegang phenomenon.

Gebhardt has made a particular study of the same subject in the case of ‘insectst. He declares, for instance, that the banded wings of Papilio podalirius are precisely imitated in Liesegang’s experiments; that the finer markings on the wings of the Goatmoth (Cossus ligniperda) shew the double arrangement of larger and of smaller intermediate rhythms, likewise manifested in certain cases of the same kind; that the alternate banding of the antennae (for instance in Sesia spheciformis), a pigmentation not concurrent with the segmented structure of the antenna, is explicable in the same way; and that the “ocelli,’ for instance of the Emperor moth, are typical illustrations of the common concentric type. Darwin’s well-known disquisition* on the ocellar pattern of the feathers of the Argus Pheasant, as a result of sexual selection, will occur to the reader’s mind, in striking contrast to this or to any other direct physical explanationt. To turn from the distribution of pigment to more deeply seated structural characters, Leduc has shewn how, for instance, the laminar structure of the cornea or the lens is again, apparently, a similar phenomenon. ~ In the lens of the fish’s eye, we have a very curious appearance, the consecutive lamellae being roughened or notched by close-set, interlocking sinuosities; and precisely the same appearance, save that it is not quite so regular, is presented in one of Kiister’s figures as the effect of precipitating a little sodium phosphate in a gelatinous medium. Biedermann has studied, from the same point of view, the structure and development of the molluscan shell, the problem which Rainey had first attacked more than fifty years beforet; and Liesegang himself has applied his results to the formation of pearls, and to the development of bone§.