306 GROWTH IN LENGTH OF THE EMBRYO. 



Homologous probably with the hepatic cells, and equally of 

 hypoblastic origin, are the spheroidal 'secreting cells' of the 

 pancreas and other glands. The epithelium of the salivary 

 glands, though these so closely resemble the pancreas, is proba- 

 bly of epiblastic origin, inasmuch as the cavity of the mouth is 

 entirely lined by epiblast. 



The hypoblast also lines the allantois. To these parts must 

 be added the notochord and subnotochordal rod. From the 

 mesoblast are formed all the remaining parts of the body. 

 The muscles, the bones, the connective tissue and the vessels, 

 both arteries, veins, capillaries and lymphatics with their appro- 

 priate epithelium, are entirely formed from the mesoblast. 



The generative and urinary organs are entirely derived from 

 the mesoblast. It is worthy of notice that the epithelium of the 

 urinary glands, though resembling the hypoblastic epithelium of 

 the alimentary canal, is undoubtedly mesoblastic. 



From the mesoblast are lastly derived all the muscular, con- 

 nective tissue, and vascular elements, as well of the alimentary 

 canal and its appendages as of the skin and the tegumentary 

 organs. Just as it is only the epidermic moiety of the latter 

 which is derived from the epiblast, so it is only the epithelium 

 of the former which comes from the hypoblast. 



Growth in length of the Vertebrate Embryo. 



With reference to the formation and growth in length of the body of the 

 Vertebrate embryo two different views have been put forward, which can be 

 best explained by taking the Elasmobranch embryo as our type. One of 

 these views, generally held by embryologists and adopted in the previous 

 pages, is that the Elasmobranch embryo arises from a differentiation of the 

 edge of the blastoderm ; which extends inwards from the edge for some little 

 distance. This differentiation is supposed to contain within itself the rudi- 

 ments of the whole of the embryo with the exception of the yolk-sack ; and 

 the hinder extremity of it, at the edge of the blastoderm, is regarded as 

 corresponding with the hind end of the body of the adult. The growth in 

 length takes place by a process of intussusception, and, till there are formed 

 the full number of mesoblastic somites, it is effected, as in Chastopods, by 

 the continual addition of fresh somites between the last-formed somite and 

 the hind end of the body. 



A second and somewhat paradoxical view has been recently brought into 

 prominence by His and Rauber. This view has moreover since been taken 

 up by many embryologists, and has led to strange comparisons between the 



