26 PKELIMINARY ACCOUNT. [CHAP. 



these the epiblast and hypoblast constitute the primary 

 layers. 



Three similar germinal layers are found in the 

 embryos of all vertebrate and most invertebrate forms, 

 and their history is one of the most important parts of 

 comparative embryology. 



The epiblast gives rise to the epidermis, the central 

 and peripheral parts of the nervous system, and to the 

 most important parts of the organs of special sense. 

 The hypoblast is essentially the secretory layer, and 

 furnishes the whole epithelial lining of the alimentary 

 tract and its glands, with the exception of part of the 

 mouth and anus which are lined by the epiblast and 

 are spoken of by embryologists as the stomodceum and 

 proctodceum. Finally the mesoblast is a source from 

 which the whole of the vascular system, the muscular 

 and skeletal system, and the connective tissue of all 

 parts of the body, are developed. It gives in fact origin 

 to the connective-tissue basis both of the skin and of 

 the mucous membrane of the alimentary tract, and to 

 all the structures lying between these two with the 

 exceptions already indicated. It is more especially to 

 be noted that it gives rise to the excretory organs and 

 generative glands. 



Formation of the embryo. The blastoderm which 

 at first, as we have seen, lies like a watch-glass over the 

 cavity below, its margin resting on the circular germinal 

 wall of white yolk, spreads, as a thin circular sheet, over 

 the yolk, immediately under the vitelline membrane. 

 Increasing uniformly at all points of its circumference, 

 the blastodermic expansion covers more and more of the 

 yolk, and at last, reaching the opposite pole, completely 

 envelopes it. Thus the whole yolk, instead of being 



