AND ON THE DIRECT AGENCY OF THE SPERMATOZOON. 251 



ing some eggs on the dark surface, others on the under or white surface, and others 

 at the sides between the two. At the end of Jive hours and seven minutes the chamber 

 had been formed and segmentation was taking place in each egg, and on \hejifth day 

 each had produced an embryo, thus appearing to show that it is of little consequence, 

 with reference to fecundation, as to which part of the egg the fecundatory agent is 

 applied to. But this conclusion is subject to certain conditions, as I shall proceed to 

 explain. 



9. PORTION OF THE YELK MOST SUSCEPTIBLE OF FECUNDATION. 



On attentively examining the preceding experiments, it seemed difficult to under- 

 stand how it happened that when similar quantities of the same fluid were applied by 

 the pin-head to different eggs of the same brood, at the same time, and the whole of 

 them placed in exactly similar conditions in regard to heat, light and aeration, that 

 some should become fecundated and others not, if, what appeared to be shown in the 

 last detailed experiment, that the egg can be fecundated equally well by the appli- 

 cation of the fecundatory agent to any part of its surface, be strictly correct. There 

 at first seemed to be no other way of accounting for the differences in the results, 

 except on the hypothesis, either that it was due to some insusceptibility in the entire 

 egg itself, which thus presupposed great imperfection in each brood of eggs, a con- 

 clusion at variance with observation, or that it resulted from the mode in which 

 fecundation was attempted. 



During the past season (1852) I have put the question, thus raised to a more rigid 

 test, in order to learn whether the egg really is as susceptible of fecundation in one 

 part of its structure as in another. 



In order to obtain some decisive results I repeated my experiments on three sepa- 

 rate occasions, with eggs and fluid from three pairs of frogs, at nearly similar tempe- 

 ratures of the atmosphere. On the first occasion, April 5th, the temperature was 

 54FAHR., on the second, the following day, it was 55FAHR., and on the last occa- 

 sion, April 9th, it was 53 FAHR. 



In all the preceding experiments the eggs had been allowed to remain in exactly 

 the same position, at the time of applying the fluid, as that in which they had passed 

 from the body of the frog ; so that sometimes the dark, and sometimes the white 

 surface was uppermost, and at other times inclined more or less to either side, and 

 occasionally the two were perfectly horizontal. When the fluid was applied by means 

 of the pin's-head to the part desired, some portion of it necessarily flowed over other 

 parts, if the surface which received it happened to be inclined to one side or the 

 other. 



But in my more recent investigations care was taken to place every egg in a per- 

 fectly vertical position, before attempting to fecundate it, so that in one set of eggs 

 the central part of the dark surface of the yelks was uppermost, and in another the 

 centre of the white surface. These precautions led to very definite results. In the 



MDCCCLIII. 2 L 



