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 un- 
explained. Although they merit the greatest attention and re- 
quire 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 le 
side by side unconnectedly, ike productions which are merely 
capable of classification in natural history, according to simi- 
larity of form; they are united by a common bond, the 
similarity of their formative principle, and they may be com- 
pared 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 im accordance with this physiological arrange- 
ment, 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 neces- 
sarily 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, inas- 
much 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 germi- 
nal 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 
