MAMMALIA. 



217 



vesicle. The greater 

 part of its walls are 

 formed of a single row 

 of flattened epiblast 

 cells; while the hypo- 

 blast cells form a small 

 lens -shaped mass at- 

 tached to the inner side 

 of the epiblast cells 

 (fig- 135). 



In the Vespertilionidee 

 Van Beneden and Julin have 

 shewn that the ovum under- 

 goes at the close of seg- 

 mentation changes of a more 

 or less similar nature to 

 those in the rabbit ; the 

 blastopore would however 

 appear to be wider, and to 

 persist even after the cavity 

 of the blastodermic vesicle 

 has commenced to be de- 

 veloped. 



FIG. 135. RABBIT'S OVUM BETWEEN 7090 



HOURS AFTER IMPREGNATION. (After E. van 



Beneden.) 



bv. cavity of blastodermic vesicle (yolk-sack) ; 

 ep. epiblast ; hy. primitive hypoblast ; Z/. mu- 

 cous envelope (zona pellucida). 



Although by this stage, which occurs in the rabbit between 

 seventy and ninety hours after impregnation, the blastodermic 

 vesicle has by no means attained its greatest dimensions, it has 

 nevertheless grown from about 0x39 mm. the size of the ovum 

 at the close of segmentation to about 0*28. It is enclosed by a 

 membrane formed from the zona radiata and the mucous layer 

 around it. The blastodermic vesicle continues to enlarge rapidly, 

 and during the process the hypoblastic mass undergoes im- 

 portant changes. It spreads out on the inner side of the epi- 

 blast and at the same time loses its lens-like form and be- 

 comes flattened. The central part of it remains however thicker, 

 and is constituted of two rows of cells, while the peripheral part, 

 the outer boundary of which is irregular, is formed of an im- 

 perfect layer of amoeboid cells which continually spread further 

 and further within the epiblast. The central thickening of the 

 hypoblast forms an opaque circular spot on the blastoderm, 

 which constitutes the commencement of the embryonic area. 



