84 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



two organs not being homologous that they are not developed 

 from the same layer. It renders the task of tracing out the 

 homologies from development much more difficult than if the 

 ordinary view of the invariable correspondence of the three 

 layers throughout the animal kingdom be accepted. Although 

 I do not believe that this correspondence is invariable or exact, 

 I think that we both find and should expect to find that it is, 

 roughly speaking, fairly so. 



Thus, the muscles, internal skeleton, and connective tissue 

 are always placed in the adult between the skin (epidermis) and 

 the epithelium of the alimentary canal. 



We should therefore expect to find them, and, as a matter 

 of fact, we always do find them, developed from a middle layer 

 when this is present. 



The upper layer must always and does always form the 

 epidermis, and similarly the lower layer or hypoblast must form 

 a part of the epithelium of the alimentary canal. A full dis- 

 cussion of this question would, however, lead me too far away 

 from my present subject. 



The only other point of interest which I can touch on in 

 this stage is the commencing closure of the alimentary canal 

 in the region of the head. This is shewn in PI. 3, figs. 6a, 6b, 

 jb, n, a. From these figures it can be seen that the closing 

 does not take place as much by an infolding as by an ingrowth 

 from the side walls of the alimentary canal towards the middle 

 line. In this abnormal mode of closing of the alimentary canal 

 we have again, I believe, an intermediate stage between the 

 mode of formation of the alimentary canal in the Frog and 

 the typical folding in which occurs in Birds. There is, how- 

 ever, another point in reference to it which is still more inter- 

 esting. The cells to form the ingrowth from the bottom (ven- 

 tral) wall of the alimentary canal are derived by a continuous 

 fresh formation from the yolk, being formed around the nuclei 

 spoken of above (vide p. 63 et seq.). All my sections shew 

 this with more or less clearness, especially those a little later 

 than fig. 6b, in which the lower wall of the alimentary canal is 

 nearly completed. This is the more interesting since, from the 

 mode of formation of the alimentary canal in the Batrachians, 

 &c., we might expect that the cells from the yolk would take 



