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 



FIG. 18. Gastrulation of amphioxus (modified from Hatschek). 



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



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

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

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



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