988 



OP GENERATION. 



the germinal vesicle disappears at or about the time of fecundation ; but its dis- 

 appearance is not a result of fecundation, since it also takes place in the unim- 



Cleaving of the yelk after fecundation : A, B, c (from Kolliker), ovum of Ascaris nigrovenosa; D and E, that 

 of Ascaris acuminata (from Bagge). 



pregnated egg, in consequence (it may be presumed) of the completion of its 

 term of life, and of those operations which it was developed to perform. Its 

 place is seen to be occupied at an early period after fecundation, by a new and pe- 

 culiar cell, the origin of which is obscure, but the destination of which is most 

 important ; for it is by the duplicative subdivision of this cell, first into 2, then 



Fig. 261. 



A. Ovum of a Bitch, from the Fallopian tuhe, half an inch from its opening into the uterus, showing the 

 sona pellucida with adherent spermatozoids, the yelk divided into its first two segments, and two small gran- 

 ules or vesicles contained with the yelk in the cavity of the zona. B. Ovum of a bitch from the lower ex- 

 tremity of the Fallopian tube : the cells of the tunica granulosa have disappeared : the yelk is divided into 

 four segments, c. Ovum of bitch from the lower extremity of the Fallopian tube, in a latter stage of the 

 division of the yelk. D. An ovum from the uterus : it is larger, the zona thicker, and the segments of the 

 yelk are very numerous. E. Ovum from the lower extremity of the Fallopian tube burst by compression : 

 the segments of the yelk have partly escaped, and in each of them a bright spot or vesicle is visible. 



into 4, then into 8, and so on, and by the metamorphoses which its progeny 

 undergo, that the whole embryonic fabric is gradually evolved. Hence this cell 

 may be termed the embryo-cell. 1 At the same time a peculiar change begins to 



1 The embryo- cell has not yet been clearly made out in the Mammalian ovum ; but from 

 the conformity of the subsequent appearances to those which are seen in the ova of the 

 lower animals, there is every reason to believe that the formation of either a complete cell, 

 or of a nucleus having the same essential endowments, is a preliminary to the cleavage of 

 the yelk. 



