TEXT-BOOK OF EMBRYOLOGY. 



cavity becomes triangular in cross section in this region, the change in shape 

 being regarded as due to the formation of the primitive streak (Fig. 63, B). 

 It is interesting to note here that development up to this stage has re- 

 quired nine days out of the 21-23 days of gestation in the white rat. The 



exect. 



FIG. 62. Cross sections of an egg-cylinder of the white rat, 8 days and 17 hours after insemina- 

 tion. Huber. A, section which corresponds to a level just above the two crosses in Fig. 

 61, D. B, section which corresponds to a level about half an inch below the two crosses in 

 Fig. 61, D. ex.ect., extraembryonic ectoderm; p.ect., transitory ectoderm (original wall 

 of blastocyst); pr.c., proamniotic cavity; pr.emb.ect., primary embryonic ectoderm; 

 Pr.emb.ent., primary embryonic entoderm; v.ent., visceral entoderm. 



slow progress of the early cleavage stages is regarded as due to the lack 

 of nutriment as the ova pass through the oviduct. Even when the ova 

 reach the uterus they become imbedded very slowly in the uterine mucosa, 



pr.emb.ct 



FIG. 63 Cross sections of egg-cylinders of the white rat. A , 8 days and 1 7 hours after insemina- 

 tion; B, 8 days and 16 hours after insemination. Huber. These sections are taken through 

 the region of the primary embryonic ectoderm and would therefore correspond to levels 

 below the crosses in Fig. 61, D. mes., Mesoderm; p.ect., transitory ectoderm (original 

 wall of blastocyst); pr.c., proamniotic cavity; pr.emb.ect., primary embryonic ectoderm; 

 pr.emb.ent., primary embryonic entoderm; pr.gr., primitive groove; pr.str., primitive 

 streak; v.ent., visceral entoderm. 



probably receiving on that account a minimum of nourishment. During 

 development the original wall of the blastocyst becomes reduced to a mem* 

 brane around which maternal blood circulates as the vesicles become em- 



