THE FORMATION OF ECTODERM AND ENTODERM (GASTRULATION) 



Amphioxus and Amphibia. In these animals the larger cells at the 

 vegetal pole of the blastula either fold inward, that is, invaginate (Amphi- 

 oxus, Fig. 19), or are for the most part asymmetrically overgrown by the 

 more rapidly dividing cells of .the animal pole (amphibia, Fig. 15 G, H}. 

 Eventually the invaginating cells obliterate the blastula cavity and come in 

 contact with the outer layer of cells (Fig. 1 9) . The new cavity thus formed 

 is the primitive gut, or archenteron, and its narrowed mouth is the blastopore. 

 The outer layer of cells is the ectoderm, the inner, newly formed layer is 

 the entoderm. The entodermal cells are henceforth concerned in the 

 nutrition and metabolism of the body. The embryo is now termed a 

 Gastrula (little stomach). 



A nimal cells 



Blastoccele 



Entoderm 

 A rchenteron 



Ectoderm 



Aal cells 



FIG. 19. Gastrulation in Amphioxus. 



X about 200. 

 gastrulas. 



A, blastula; B, C, early and late 



Reptiles and Birds/ The germinal disc, or blastoderm, in these 

 animals lies like a cap on the surface of inert yolk (Fig. 3). Since the 

 enormous amount of yolk makes gastrulation as in Amphioxus and am- 

 phibians impossible, the process exhibits marked modifications. 



There appears caudally on the blastoderm of reptiles a pit-like depres- 

 sion. From this slight invagination a proliferation of cells forms a layer 

 which spreads beneath the ectoderm. The inner layer, originating in this 

 manner, is the entoderm, and the region of the pit, where ectoderm and 

 entoderm are continuous, is the blastopore. In Fig. 2 1 A these changes are 

 complete. 



In birds, the caudal portion of the blastoderm is rolled or tucked under, 

 the inner layer formed in this way constituting the entoderm. The 

 marginal region where ectoderm and entoderm meet bounds the blastopore, 

 while the space between entoderm and yolk is the archenteron. 



Mammals. As in cleavage, so also in gastrulation the mammalian 

 ovum exhibits a modified behavior indicative of an ancestral yolk-rich 

 condition. Cells on the under surface of the inner cell mass become 

 arranged in a definite sheet, the entoderm, which rapidly spreads and lines 



