740 GENERATION AND DEVELOPMENT. 



this time she was thrice covered by horses, and every time 

 the foal she bore had still distinct, though decreasing, 

 marks of the quagga; the peculiar characters of the 

 quagga being thus impressed not only on the ovum then 

 impregnated, but on the three following ova impregnated 

 by horses. It would appear, therefore, that the constitu- 

 tion of an impregnated female may become so altered and 

 tainted with the peculiarities of the impregnating male, 

 through the medium of the foetus, that she necessarily 

 imparts such peculiarities to any offspring she may sub- 

 sequently bear by other males. Of the direct means by 

 which a peculiarity of structure on the part of a male is 

 thus transmitted, nothing whatever is known. 



DEVELOPMENT. 



Changes in the Ovum previous to the Formation of the Embryo. 



Of the changes which the ovum undergoes previous to 

 the formation of the embryo, some occur while it is still in 

 the ovary, and are apparently independent of impreg- 

 nation : others take place after it has reached the Fallo- 

 pian tube. The knowledge we possess of these changes 

 is derived almost exclusively from observations on the 

 ova of mammiferous animals, especially the bitch and 

 rabbit : but it may be inferred that analogous changes 

 ensue in the human ovum. 



Bischoff describes the yelk of an ovarian ovum after 

 coitus as being unchanged in its characters, with the single 

 exception of being fuller and more dense; it is still 

 granular, as before, and does not possess any of the cells 

 subsequently found in it. The germinal vesicle always 

 disappears, sometimes before the ovum leaves the ovary, 

 at other times not until it has entered the Fallopian tube ; 

 but always before the commencement of the metamorphosis 

 of the yelk. 



As the ovum approaches the middle of the Fallopian 

 tube, it begins to receive a new investment, consisting of 



