THE OVUM IN THE AMPHIBIA. 193 



less will be the chance of its impregnating the ovum, and the less will the ovum 

 become susceptible of impregnation even by the most healthy and vibratile sperma^ 

 tozoa. 



The extent and rate of expansion of the envelope of the Frog's egg, during the first 

 half-hour it remains in water, very nearly coincide with the diminution of the fitness 

 of the ovum to become fecundated. This is shown by observing the rate of expansion 

 of the envelope during the first fifteen minutes of submersion, and then testing the 

 fitness of the ovum, by experiment, during a similar period. 



At the moment when the ovum is expelled from the body, the envelope is merely a 

 thin gelatinous layer, its entire diameter being equal only to about one-sixth of the 

 diameter of the yelk. After it has been one minute in water, and begun to imbibe 

 and expand, it is then equal to about one-fourth of the diameter of the yelk. At the 

 end of two minutes it is enlarged to one-third, and in three minutes to one-half the 

 diameter of this body. In four minutes it exceeds three-fifths, and in six minutes 

 two-thirds, and it continues to imbibe fluid and expand at the same rate, until, at from 

 ten to fifteen minutes, it very nearly equals in thickness the whole diameter of the 

 yelk ; and at half an hour (fig. 9) it is one-fourth greater than this. PREVOST and 

 DUMAS* noticed the expansion of the envelope during the first six hours, but entirely 

 overlooked the rate of expansion during the most important period, the first hour, 

 and noticed only the general fact that the diameter of the envelope, at the end of the 

 first hour and a half, was as 5 to 2 - 5 at the time of spawning, and that it had nearly 

 acquired its full size at the end of three hours. My own observations agree with this 

 latter statement. The expansion of the envelope is greatly retarded at the end of 

 the third or fourth hour, until after cleavage of the yelk has taken place, when it 

 again proceeds, but much more slowly than at first. If then we bear in mind the 

 rate of expansion of the envelope during the first half-hour, the following experiments 

 will give some idea of the degree of susceptibility of the ovum to become impreg- 

 nated during that period. 



Set E. April 6, 1850. The temperature of the room, at the commencement of this 

 set of experiments, being 60 FAHR., ova were obtained from a female frog and seminal 

 fluid from a male, by the mode already mentioned ; the latter being mixed with an 

 equal quantity of water. 



I may here remark, that the ova in each of this set of experiments were placed 

 in nearly similar quantities of water, and that as it had been shown in the experi- 

 ments A, B, C and D (p. 190), that segmentation of the yelk proves the ovum to 

 have been impregnated, although, as we shall hereafter find, not always sufficiently 

 so as to produce the embryo, I adopted this as a fair test of the susceptibility of 

 the ovum. I may here also mention, that although the date of making the several 

 experiments detailed in this paper is recorded, it has been necessary, for reasons 

 that will be obvious, to disregard the order of time at which the several sets were 



* Loc. cit., vol. ii. p. 108. 

 MDCCCLI. 2 C 



