X.] IMPKEGNATION. 311 



position and undergoes a series of changes which have 

 not been fully worked out, but which probably are of 

 the same nature as those which have been observed in 

 other types (p. 17). The result of the changes is the 

 formation of one or more polar bodies, and the nucleus 

 of the mature ovum (female pronucleus). 



At certain periods one or more follicles containing a 

 ripe ovum burst 1 , and their contents are received by 

 the fimbriated extremity of the Fallopian tube which 

 appears according to Hensen to clasp the ovary at the 

 time. The follicle after the exit of the ovum becomes 

 filled with blood and remains as a conspicuous object on 

 the surface of the ovary for some days. It becomes 

 eventually a corpus luteum. The ovum travels slowly 

 down the Fallopian tube. It is still invested by the 

 zona radiata, and in the rabbit an albuminous envelope 

 is formed around it in its passage downwards. Im- 

 pregnation takes place in the upper part of the Fallo- 

 pian tube, and is shortly followed by the segmentation, 

 which is remarkable amongst the Amniota for being 

 complete 2 . 



The entrance of the spermatozoon into the ovum 

 and its subsequent fate have not been observed. Van 

 Beneden describes in the rabbit the formation of the 

 first segmentation nucleus (i.e. the nucleus of the ovum 

 after fertilization) from two nuclei, one peripheral and 

 the other ventral, and deduces from his observations 



1 So far as is known there is no relation between the bursting of 

 the follicle and the act of coition. 



2 It is stated by Bischoff that shortly after impregnation, and 

 before the commencement of the segmentation, the ova of the rabbit 

 and guinea-pig are covered with cilia and exhibit the phenomenon of 

 rotation. This has not been noticed by other observers. 



