272 MR. NEWPORT ON THE IMPREGNATION OF THE OVUM IN THE AMPHIBIA 
middle of the dark hemisphere of the yelk, and which, possibly, may be of less speci- 
fic gravity than the contents of the light-coloured hemisphere. 
The facility with which the yelk can thus be made to rotate, and its flattened 
surface, and the clear space, or chamber above it,-commenced within one hour after 
the first encoonter of spermatozoa with the egg, —can be brought beneath any portion 
of the vitellary membrane into which spermatozoa have penetrated,— thus enabled 
me to observe, within a comparatively short period after fecundation, whether any of 
those bodies are at that time in the act of passing through it. But although no 
spermatozoa could then be detected within the vitellary cavity, it must be borne in mind 
that these observations were made after the commencement of the formation of the 
clear space, or respiratory chamber, the consequence of a shrinking and depression of 
the surface of the dark hemisphere of the yelk as the result of impregnation, so that 
these negative observations did not afford any ;?roo/ that penetration by the sperma- 
tozoon had not previously taken place. Indeed, all circumstances considered, there 
seems reason to believe that impregnation is effected very quickly after the 
encounter of the spermatozoon with the egg, as appeared to be shown in my former 
experiments with solutions of potass^. It is probable that it is commenced, as then 
suggested, within a few seconds, or at most a very few minutes, after such encounter ; 
as in one instance of artificial impregnation in some observations made subsequently 
to those above detailed, I detected a spermatozoon in contact with the vitellary 
membrane, within one minute after the impregnating bodies had been supplied to an 
egg beneath the microscope. Certainly I believe that it is commenced, and probably 
completed, within the first half -hour ; because, after that lapse of time, I have not 
yet been able to detect any change of place or position of the spermatozoa which 
have passed into the envelopes of the egg ; whether they may have already arrived at 
and become partially imbedded in the vitellary membrane, or whether they are still 
contained in the clear substance of the outer coverings, their force of penetration 
being exhausted before arrival at the membrane. After the lapse of half an hour, 
and frequently of a much shorter time, the caudal portion of the body of a sperma- 
tozoon which has penetrated into the envelopes becomes looped on itself, and this is 
a certain indication of the death of the object. 
As no spermatozoa were seen, in these observations f, within the vitellary cavity at 
the early period above stated, it was not to be expected that they were likely to be 
observed at a later ; although the eggs which had been the subject of these investiga- 
tions, and which had been fecundated naturally, were continued to be watched until 
segmentation of the yelks commenced. This took place in the two eggs, in which 
alone, out of the fifteen first examined, spermatozoa were detected, at the expiia- 
tion oifive hours and twenty minutes. But this was much earlier than in the gieat 
mass of eggs, of which these were a part; and was due to their having been exposed 
to a higher temperature, owing to heat radiated from my own body dining theii 
* Philosophical Transactions, 1851. t See the preceding note. 
