GENERAL RETROSPECT. 1G7 



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 lie 

 side by side unconnectedly, like 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 in 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 docs 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 



