THE GEEMINAL LAYERS. 



23 



indirectly, from tlie hypoblast. Though appearing after the 

 other two germinal layers, the mesoblast grows very rapidly, and 

 in the higher animals forms a larger part of the embryo than the 

 other two layers together. 



The two primary germinal layers, epiblast and hypoblast, 

 occur, and with essentially similar relations, in all groups of 

 Metazoa,from sponges up to mammals. The middle germinal 

 layer, or mesoblast, presents far greater variations, and it is by 



Fig. 8. 



Fig. 9. 



Figs. 8, 9. — Vertical and horizontal sections of early larval stages 

 of Amphioxns. x 220. (After Hatsohek.) 



CE, commencing mepoblastic outgrowtli. E, epiblast. Gj archenteron. H, hypo- 

 blast. lirP, neural fold. WT, ueurenteric canal, leading from neural tube to archenteron. 

 PC, polar mesoblast cell. 



no means clear that all the structures spoken of as mesoblastic 

 in the different groups of animals have any real community of 

 origin or relations. In Sponges and Coelenterates a mesoblastic 

 layer cannot be said to exist, but in all other groups of Metazoa 

 it is present. 



The three germinal layers together make up the whole of 

 the embryo, and from them all parts of the adult animal are 

 derived : the principal organs and parts to which the layers 

 give origin respectively are as follows. 



The epiblast, or outer layer, gives rise to the epidermis, cover- 

 ing the body generally ; and to the various organs derived from 

 the epidermis. Of these, the more important are : — the nervous 

 system, both central and peripheral ; the olfactory and auditory 

 epithelium, the retina and lens of the eye, and the other organs 

 of sensation ; the epithelial lining of the mouth and anus ; the 

 ■pineal and pituitary bodies ; the enamel of the teeth ; the hairs, 



