GENERAL RETROSPECT 167
of the embryos of pigs, I found many cells in the cylinders, which were so large as to occupy almost the entire thickness of the canal. In other cylinders, the cellular layer, which was subsequently to line their walls, was formed, but the cavity was filled with very pale transparent cells, which could be pressed out from the free end of the tube.)
These and similar phenomena may remain for a time unexplained. Although they merit the greatest attention and require further investigations, we may be allowed to leave them for a moment, for history shows that in the laying down of every general principle, there are almost always anomalies at first, which are subsequently cleared up.
The elementary particles of organisms, then, no longer lie side by side unconnectedly, like productions which are merely capable of classification in natural history, according to similarity of form; they are united by a common bond, the similarity of their formative principle, and they may be compared together and physiologically arranged in accordance with the various modifications under which that principle is exhibited. In the foregoing part of this work, we have treated of the tissues in accordance with this physiological arrangement, and have compared the different tissues with one another, proving thereby, that although different, but similarly formed, elementary parts may be grouped together in a natural-history arrangement, yet such a classification does not necessarily admit of a conclusion with regard to their physiological position, as based upon the laws of development. Thus, for example, the natural-history division, "cells," would, in a general sense, become a physiological arrangement also, inasmuch as most of the elementary parts comprised under it have the same principle of development; but yet it was necessary to separate some from this division ; as, for instance, the germinal vesicle, all hollow cell-nuclei, and cells with walls composed of other elementary parts, although the germinal vesicle is a cell in the natural-history sense of the term. It does not correspond to an epithelium-cell, but to the nucleus of one. The difference in the two modes of classification was still more remarkable in respect to fibres. The mode of their origin is most varied, for, as we saw, a fibre of areolar tissue
