354 



THE BABBIT. 



In the spreading of the smaller cells over the larger ones, th& 

 rabbit and the frog appear to agree ; but the details of the process 



Fig. 138. 



Fig. 139. 



Fig. 188. — A Babbit's Ovum seventy hours after copulation, taken from th& 

 lower end of the oviduct just before entering the uterus, and showing the 

 condition at the close of segmentation. (After Van Beueden.) x 200. 



Fig. 139. — A Rabbit's Ovum seventy-five hours after copulation, taken from the 

 uterus, and showing the first stage in the formation of the blastodermic 

 vesicle. (After Van Beneden.) x 200. 



CC, outer layer of cells. CD, inner mass of cells. CV, cavity of blastodermic vesicle. 



have not been accurately determined in the rabbit's ovum, and 

 it is doubtful how far the correspondence is a real one. 



2. The Blastodermic Vesicle. 



On entering the uterus, at the end of the third day, the ovum 

 has the structure shown in Fig. 138 and described above. It is- 

 spherical in shape, with a diameter averaging 0-09 mm., i.e. is- 

 slightly smaller than the unfertilised egg. It is surrounded by 

 the vitelline membrane and zona radiata as before ; and outside 

 the latter are the concentric layers of albumen, which are de- 

 posited round the egg during its passage along the-oviduct, and 

 which have a total thickness of about 0-1 mm. 



From three to nine ova are usually discharged from the ovary 

 at each period of ovulation. These enter the uterus almost 

 simultaneously, and at first lie close together at its proximal 

 end. As development proceeds they gradually become spread out 

 along the uterus, at approximately equal intervals ; each ovum 

 lying in a special dilatation of the uterus, to the wall of which 

 it becomes attached during the eighth day. 



Very shortly after the egg enters the uterus, and in some 

 cases before it leaves the oviduct, the smaller outer cells grow 



