88 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



the morula stage by the collection of fluid within the mass of 

 cells. The cells are then seen to be arranged in two sets— (a) a 

 layer lining the zona radiata and forming the wall of the vesicle 



Zona radiata 



germinal cells. 

 Rauber's layer 



Fio. 66. — Diagrammatic section of a Blastodermic Vesicle. 



(Eauber's layer) (Fig. 66), and (6) a group of granular cells 

 within the cavity attached to Eauber's layer at the embryogenic 

 pole of the vesicle (Fig. 66). In Vertebrates with huge stores 

 of yolk in their ova, such as birds have, the vesicle is filled by 

 yolk-bearing cells, continuous with Eauber's cells at the vegeta- 

 tive pole, opposite to the granular cells. 



origin of mesobl. 



primitive groove 



Zona radiata 

 epiblast 

 hypoblast 

 arohenteron 



Fig. 67. -A diagrammatic section of a Bilaminar Blastoderm made across the 

 primitive streak. 



3. The Bilaminar Blastoderm. — This stage is produced by 

 the growth of the group of granular germinal cells within Eauber's 

 layer. At first they arrange themselves in two layers — an outer 



