SEGMENTATION OF THE FERTILIZED OVUM 



formed is not known. It is attached only to the cells of the germinal area. 

 Although this stage has not been observed in the human embryo it is probable 

 from the structure of the youngest known human embryo that an entodermal 



vesicle is formed in much the same 

 way as in Tarsius. 



Gastrulation in Reptiles and 

 Birds. After the formation of the 

 primary or yolk entoderm in reptiles 

 and birds, a process of invagination 

 takes place which has been compared 

 to gastrulation in Amphioxus and 

 Amphibia. In the lizard after the 





FIG. 21. Two germ-discs of hen's egg in the 

 first hours of incubation (after Roller in Heisler) : 

 df, area opaca; hf, area pellucida; s, crescent; sk, 

 crescent-knob; es, embryonic shield; pr, primitive 

 groove. 



B 



primary entoderm has developed by 

 delamination a curved depression is 



formed at the posterior border of the germinal area. There is a true invagina- 

 tion of cells at this point (Fig. 20). The cells grow cephalad and contain an 

 invagination cavity. Later the floor of the cavity fuses with the yolk entoderm 

 and disappears. The cells of the roof persist as the dorsal or notochordal plate. 

 The crescentic depression, at which point 

 invagination occurs, may be compared 

 to the blastopore of Amphioxus. The 

 invagination cavity is the gastrula cavity 

 or archenteron and the dorsal plate 

 represents the entodermal roof of the 

 gastrula cavity. 



In the bird, invagination takes place 

 at a crescentic groove (Fig. 21), but the 

 ingrowing cells form a solid plate, at 

 first without an invagination cavity. 

 The result is the same, however, the 



formation of a notochordal plate which lies beneath the ectoderm. The crescentic 

 groove is interpreted as representing the blastopore (Fig. 19). 



Formation of the Primitive Streak in Birds. The germinal area increases 

 in size by growth about its periphery, new cells constantly being formed here until 

 eventually the germinal area surrounds the yolk. According to the interpreta- 

 tion of Duval and Hertwig, as the periphery of the germinal area extends itself, 

 a middle point in the cranial lip of the crescentic groove remains fixed while the 



FIG. 22. Diagram elucidating the forma- 

 tion of the primitive groove (after Duval). 

 The increasing size of the germ-disc in the 

 course of the development is indicated by dot- 

 ted circular lines. The heavy lines represent 

 the crescentic groove and the primitive groove 

 which arises from it by the fusion of the edges 

 of the crescent (Heisler). 



