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 {425} 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[429].” 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 mollusc 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;

Fig. 201. A “crust” of close-packed cal­car­eous con­cre­tions, pre­cip­i­tated at the sur­face of an al­bum­i­nous so­lu­tion. (After Har­ting.)Fig. 202. Ag­gre­gated cal­co­spher­ites. (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])[430], 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 {426} dispersion medium). In accordance with a rule first recognised by Ostwald[431], 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[432]. 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 liquid 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.

Fig. 203. (After Harting.)

Where we have simple aggregates of two or three cal­co­sphe­rites, the resulting figure is precisely that of so many contiguous soap-bubbles. 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 {427} 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[433] is not obvious.