THE OVUM IN THE AMPHIBIA. 
193 
less will be the chance of its impregnating the ovum, and the dess 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, 1 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, 
2 c 
MDCCCLI. 
