38 



SEGMENTATION OF THE FERTILIZED OVUM 



formed by a process termed gastrulation. The larger cells at the vegetal 

 the blastula either fold inward (invaginate) or are overgrown by the more 

 dividing micromeres. Eventually the invaginating cells 

 obliterate the blastula cavity and come into contact 

 with the outer layer of cells (Fig. 18). The new cavity 

 formed is the primitive gut, or archenteron. The mouth 

 of this cavity is the blastopore. The outer layer of cells 

 is the ectoderm, the inner, newly formed layer is the ento- 

 derm. The entodermal cells are henceforth concerned in 

 the nutrition and metabolism of the body. The embryo 

 is now termed a Gastrula (little stomach). 



The Origin of the Entoderm in Reptilia, Birds and 

 Mammals. — Here the entoderm arises in quite a differ- 

 ent manner. Instead of a process of gastrulation by in- 

 vagination of cells we have first a process of delamination. 

 Cells are split off or delaminated from the under side of 

 the germinal area, arrange themselves in a definite inner 

 layer, and thus the yolk entoderm is formed. This layer 

 is already apparent in a longitudinal section through the 

 germinal area of a chick (Fig. 19) . In mammals like the 



pole of 

 rapidly 



fej 



.••i 



Fig. 18. — Gastrulation of amphioxus (modified from Hatschek). 



A, Blastula; az, animal cells; vz, vegetative cells; fh, cleavage-cavity. 



B, Beginning invagination of vegetative pole. C, Gastrula stage, the 

 invagination of the vegetative cells being complete; ah, outer germ- 

 layer; ik, inner germ-layer; ud, archenteron; u, blastopore (Heisler). 



c 

 c 

 o 

 pq 





