9 6 4 



MATURATION OF THE OVUM. 



vesicle is known as the vascular area. The discovery of an analogous 

 formation in the meroblastic ovum, as, for example, that of birds, 

 has been attended with no little difficulty. In such ova only partial 

 cleavage takes place, that is only the white yolk in the region of the 

 cock's treadle undergoes division into many blastomeres in the process 

 of segmentation as a result of processes in other respects analogous to 

 those in the ova of mammals. The cells thus resulting form on the 

 surface of the yolk the germinal membrane; and they later on become 

 arranged into two superposed, thin, circular layers or germinal plates. 

 The upper layer (ectoblast) is the larger and contains smaller, paler 

 cells. The lower layer (hypoblast), which at first is not arranged con- 

 tinuously, is smaller, and its cells are larger and darkly granular. 



Observation of germinal plates during the first hours of hatching 

 permits the recognition of conditions indicative of a formative process 

 in the development of the hypoblast analogous to that occurring in 



FIG. 



E ^ 



374- A, Germinal plate of hen's egg in the first hours of incubation (after Roller); df, dark germinal area; 



/, clear germinal area; *, rudimentary embryo; u, point from which the hypoblast is formed by invagina- 



tion, or the blastopore (primitive mouth) becomes sickle-shaped below (<r). B, Somewhat older preparation; 



*"i P"*" 



Duva 



C, 



:horda dorsalis. 



holoblastic ova. A formation corresponding to the primitive mouth 

 has been encountered also on the germinal plate of the bird (Fig. 374, 

 A, u) This at first is short and is expanded in its lower area into 

 the shape of a sickle. This blastopore, gradually becoming longer, 

 levelops into the primitive streak (B, C, pr), which undoubtedly is 

 comparable with that of mammals. That in the ova of birds the hypo- 

 blast must likewise be considered to have resulted from invagination 

 the blastopore is rendered probable by the study of a longitudinal 

 ction of the two germinal plates at this first period. Fig. 374 E 

 represents such a sagittal section of the germinal plate from a nightin- 

 ihe lower germinal plate (h) appears to be pushed out 

 trom the blastopore (u) under the ectoblast. Both plates rest upon 

 the cavity of the archenteron ( c ) filled with fluid 



