CHAPTER VI 



GASTRULATION, FORMATION OF GERM LAYERS, AND 

 DEVELOPMENT OF THE ENTODERM 



In simple cases, among animals' eggs where there is but little yolk, 

 the cleavage cells (blastomeres) remain associated in a single mass, which, 

 because of its appearance, has been called the "mulberry," or "morula," 

 stage. The morula may become a single-layered hollow sphere of cells 



(g) 



f$1> 



V.^ 





C D 



Fig. 38. — An animal egg. A, unfertilized egg. B, four-cell stage. C, blastula stage. 

 D, gastrula stage. 



(blastula) the cavity of which is the segmentation cavity, or blastocoele 

 (Fig. 38C). One side of the hollow blastula then becomes pushed 

 inward, or invaginated, as if one side of a hollow rubber ball were dimpled, 

 resulting in a cup-hke structure (Fig. 3SD). The process of invagination 

 is termed " gastrulation " ; the cup with its double wall, the "gastrula." 

 The cavity of the gastrula is the primitive digestive cavity (archenteron) ; 

 the opening is the blastopore. Of the two layers resulting from the 

 invagination of the blastula, the outer one forms the ectoderm, or epi- 



65 



