AND ON THE DIRECT AGENCY OF THE SPERMATOZOON. 253 



becoming affected. In like manner when the fluid is applied directly to any given 

 part of the surface artificially, when the egg is exposed in an empty cell, and the 

 fluid is allowed to gravitate, before the egg is immersed in water, it is of little con- 

 sequence what part is first touched with it, since it may spread over the surface to 

 the most susceptible portion ; there being in reality one part of the egg which is 

 more susceptible than another. The extremes of susceptibility and insusceptibility 

 appear to be, on the one hand the centre of the dark, or future upper surface of the 

 yelk, and on the other that of the white or under surface*. 



That these conclusions, deduced from experiment, are correct, seems to be shown 

 in the fact, formerly pointed out-f-, that the germinal vesicle originally, and its pro- 

 geny subsequently, at the time of fecundation, occupies the centre, not of the entire 

 egg itself, but of the upper or dark hemisphere of the yelk ; and this possibly may be 

 the structure to be fecundated. 



These facts deserve consideration in connexion with that of the first changes in the 

 yelk taking place in this hemisphere; first, in the shrinking of the yelk at this part; 

 the formation of the chamber above it, subsequent to fecundation; the existence of a 

 canal in its centre, in the margin of which the cleavage of the yelk commences ; and, 

 lastly, in this being the portion of the yelk in which the first lineaments of the embryo 

 become apparent. 



In connexion with these remarks, it is but just to mention that a former observer, 

 Dr. MARTIN BARRY, has distinctly referred to the changed germinal vesicle as being 

 the structure in which the future embryo, in the Mammalia, originates!. 



10. EXCESS OF FECUNDATORY INFLUENCE. 



At the time of commencing the foregoing experiments with small quantities of 

 fluid, I made others of a directly opposite character with an excess of the same. 

 Four eggs were placed each in a separate cell, and within one minute afterwards the 

 cells were filled with a mixture composed of equal parts of seminal fluid and water. 

 This was half an hour after the fluid had been obtained. I had expected, from the 

 vast quantity of spermatozoa present in the fluid, that segmentation of the yelk in 

 these eggs would have taken place very quickly; but to my great surprise, although 

 the eggs were preserved in a temperature of 60 FAHR. no segmentation whatever had 

 commenced in them at the expiration of twelve hours ! at which time an abundance 

 of spermatozoa adhered to every part of the surface of their envelopes. But no 

 chamber was formed in either of these eggs, as neither of them had been fecundated. 



* During the present year I have most fully confirmed these views by numerous experiments, and have proved 

 to my full satisfaction that the white surface of the egg is least, and perhaps not at all, susceptible ; while of 

 the dark surface, that the centre, which is occupied by the canal, and its contents, is the fertilizable part. G. N., 

 April 18, 1853. 



t Philosophical Transactions, 1851, p. 176. J Ibid. 1840. 



2 L 2 



