362 THE KABBIT. 



of the third day it passes into the uterus, and undergoes further 

 changes, consisting chiefly in the establishment of the germinal 

 layers, and in preparations for the attachment of the ovum to 

 the uterus. Up to the end of the seventh day the ovum lies 

 freely in the uteras, and there is no trace of the embryo, 

 which does not commence to form until the early part of the 

 eighth day. 



In estimating the age of rabbit ova, or embryos, it is custo- 

 mary to date from the time of copulation, which can always be 

 determined with precision ; and this method of computation will 

 be adopted here. As the eggs are not discharged from the ovary, 

 or fertilised, until from eight to twelve hours after this event, the 

 actual time during which developmental changes have been pro- 

 ceeding will be less than the stated periods by this amount. 



1. Segmentation of the Egg. 



Segmentation is effected while the egg is travelling down 

 the oviduct towards the uterus. During its passage it becomes 

 surrounded by a thick layer of albumen, formed of concentric 

 layers secreted by the walls of the oviduct. The egg itself, on 

 entering the uterus at the close of segmentation, is practically 

 the same size as the unfertilised egg, in reality slightly 

 smaller than this ; but owing to the layer of albumen, which 

 may be thicker than the diameter of the egg itself, it appears on 

 a superficial examination to have increased considerably in size. 



Segmentation commences, according to Van Bene'den, some 

 ten or twelve hours after fertilisation is effected, i.e. from 

 eighteen to twenty-four hours after copulation, and is con- 

 tinued during the next two days. About the seventieth hour, 

 or the end of the third day from the time of copulation, segmen- 

 tation is completed, and the ovum enters the uterus. 



In segmentation, the first cleft (Fig. 136) divides the egg into 

 two ovoid cells, which are nearly, but according to Van Beneden 

 not absolutely, equal in size. 



After a pause of about four hours, each of these cells again 

 divides into two, giving in all four cells, which from the first are 

 approximately spherical in shape. Each of these four then 

 divides, giving eight in all, of which the four derived from the 

 smaller of the first two cells are slightly smaller than the other 

 four. 



