860 



THE EABBIT. 



single layer of flattened pavement cells, thickened in their 

 centres by the nuclei, but very thin at their margins. 



Beyond the embryonal area, the lower layer, or hypoblast, has 

 extended further round the vesicle than before, so that it now 

 lines about a third of the entire vesicle ; the wall of the remain- 

 ing two-thirds still consists of a single layer of flattened cells, 

 continuous with those forming Eauber's layer. 



The seventh day. During the seventh day, and often before 

 the close of the sixth the blastodennic vesicle loses its spherical 



Fig. lU. 



Fig. 143. 



Fig. 143. — The blastodermic vesicle of a Babbit at the end of the seventh 

 day, seen from above. (Modified from Kolliker.) x 12. 

 AD, embryonal area. AG-, ^^'all of blastodermic vesicle. M, dotted line indicating 

 the boundary of the mesoblast, PS, primitive streak. 



Fig. 144. — The embryonal ai-ea of a Babbit at the middle of the eighth daj'. 

 (Modified from Kolliker.) x 12. 



NF, neural folil. WGr, neiural groove. PG, primitive groove. PS, primitive 

 .'.treak. 



shape, and becomes ellipsoidal (Fig. 143). The average dimen- 

 sions of the entire vesicle at the end of the seventh day are 

 from 4'5 to 5 mm. in length, by 3'5 to 4 mm. in width; but 

 individual specimens may considerably exceed these limits. 



The embryonal area (Fig. 143, ad) is now distinctly pyriform 

 ill outline, measuring on an average 1'5 mm. in length, by 

 1 mm. in width. Its longer diameter corresponds to the axis 

 of the blastodermic vesicle, and, as a rule, to that of the uterus 

 as well. From their relations to the embryo at a later stage, 



