570 GENERATION. [CHAP. XL. 



These researches establish the fact of the existence of one or 

 more pores or tubes passing through the coriaceous envelope, and 

 opening upon its interior. Leuckart has shewn that these pores 

 are characteristic of all insect ova.* 



In July, 1854, Dr. Ransom made some very important observa- 

 tions upon the ova of the stickleback, and demonstrated the exist- 

 ence of a funnel-shaped depression, pierced by a canal, which 

 passes through the chorion in the unimpregnated ovum. This is 

 the micropyle, through which the spermatozoa pass to the interior 

 of the ovum. " In the act of impregnation one or more (as many 

 as four have been seen) spermatozoids pass into the micropyle. 

 Actively moving spermatozoids may remain in contact with the 

 chorion for eighteen minutes at least without producing any sen- 

 sible change in the ovum, provided none of them enter the micro- 

 pyle; but when one is seen to enter, in about a quarter of a 

 minute a change is observable." f 



Dr. Ransom has found the micropyle in all the fresh-water fishes 

 which he could obtain. 



The existence of the micropyle in the mammalian ovum has not 

 yet been satisfactorily proved. Remak, however, regards certain 

 streaks existing in the zona pellucida as pores or micropyles 

 through which the spermatozoa may pass. 



Changes in the Ovum immediately succeeding. Impregnation. The 

 period of time, at which the ovum leaves the ovary, and passes into 

 the Fallopian tube, varies considerably in different animals; some- 

 times it occurs within a few hours after impregnation, while in 

 other instances, days, or even weeks, may elapse between the time 

 of coitus and the escape of the ovum from the Graafian follicle. 

 In the dog, the ovum may sometimes be found in the Fallopian 

 tube within thirty-six hours after coitus, and, at others, not until 

 ten or twelve days afterwards. In the roe-deer, four months are 

 said to elapse between the act of impregnation and the escape of 

 the ovum, according to the observations of M. Pockels. About 

 the time that the ovum leaves the ovary, the cells of the membrana 

 granulosa immediately surrounding it undergo a curious change of 

 form, becoming club-shaped; their pointed extremity being at- 

 tached to the zona pellucida, by which a stellate appearance is 

 produced (Fig. 271, B). Each cell contains a nucleus; and the 

 ovum, with these radiating cells, presents a stellate appearance. 

 The cells afterwards become round, and disappear about the time 



* Miiller's Archiv., 1855. 



t Proceedings of the Eoyal Society, vol. vii. No, 7, Nov. 23rd, 1854. 





