THE CELL-THEORY 



271 



"'B' 





be intercellular substance ; while the walls of the epithelium cavities 

 are admitted on all hands to be cell-walls. We confess ourselves 

 quite unable to find any guiding principle for this nomenclature, unless 

 it be that the toughest structure surrounding a " nucleus " is to be 

 taken as cell-wall, anything soft inside it 

 being contents, and anything external to it 

 intercellular substance ; which is hardly a 

 caricature of the vagueness which pervades 

 histological works upon this subject. This 

 results, we think, from the attempt to 

 determine the homology of the parts of the 

 tissues having been made from the exami- 

 nation of their embryonic conditions, where 

 it is often very obscure, and hardly to be 

 made out. It is another matter if we adopt 

 the " principle of continuity " of Reichert — • 

 a method of investigation which has been 

 much neglected. This principle is simply, 

 that whatever histological elements pass 

 into one another by insensible gradations 

 are homologous and of the same nature ; 

 and it is so clear and easy of application, 

 that we can but wonder at its hitherto 

 limited use. We will now proceed to 

 analyze the nature of the constituents of 

 some of the most characteristic tissues in 

 this way, starting from that of embryonic 

 cartilage, as we have described it above. 



Connective tissue occurs in two forms, — 

 which, however, pass into one another by 

 infinite gradations, — -the solid and the areo- 

 lated : of the former we may take a tendon 

 as an example ; of the latter, the loose 

 areolar tissue, which is found forming the 

 inner layer of the skin and mucous mem- 

 branes. Fig. 3 represents the junction be- 

 tween the tendo-Achillis and the cartilage 

 of the OS calcis in a young kitten. At A 



we have pure cartilage, the endoplasts lying within cavities whose 

 walls present more or less defined contours. At B, the cavities and 

 their contained endoplasts are somewhat elongated, and a faint striation 

 is obvious in the upper portion of the periplastic substance, which 



Fig. 3. 



Junction of tendo-AchiUis and 

 cartilage of the calcaneum in 

 a kitten. A, Pure cartilage ; 

 B, intermediate portion ; C, 

 tendon. It must be under- 

 stood that the transition is in 

 reality much more gradual, the 

 different stages having here 

 been approximated for the 

 sake of economizing space. 



