THE OVUM IN THE AMPHIBIA. 
187 
already taken place, in so far as regards those of the yelk, the white surface of which 
then exhibits an uniform appearance. 
Changes immediately before segmentation or cleavage of the yelk. — Segmentation 
usually commences in from four to five hours. At about one hour and a half after 
spawning, the peripheral layer of cells on the middle of the dark or uppermost por- 
tion of the yelk of the impregnated ovum becomes separated from the inner surface 
of the vitelline membrane, and this separation goes on until a broad free space 
is left between this envelope and the superior layer of yelk-cells. This space, which 
we may designate the respiratory chamber, is at first but a small area above 
the middle of the dark surface of the yelk, and is commenced above the central 
canal. It seems to be occasioned by a recedence towards the interior, or a shrinking, 
at this period, of the yelk-cells of the dark hemisphere of the egg, commencing in the 
centre of this part and extending gradually, but in a less degree, to the circumfe- 
rence. This recedence goes on until the space left between the vitelline membrane 
and the yelk is equal to about one-sixth of the diameter of the whole mass, when the 
space appears to be occupied by a very transparent fluid, interposed between the now 
depressed surface of the yelk and the vitelline membrane. In the centre of the black 
surface is the minute orifice noticed by Prevost and Dumas*, and BaerI, which leads 
into the central canal that communicated with the germinal vesicle in the ovarian 
ovum. It is in the margins of this canal that segmentation is commenced. While 
the space or chamber between the black portion of the yelk and the vitelline mem- 
brane is being formed, and from fifteen to thirty minutes before there is any sign of 
cleavage, the yelk becomes extended horizontally in a direction transverse to that in 
which the first cleft afterwards takes place, and assumes a transitory obtuse oval 
form, which it retains until the yelk begins to divide. The division, as correctly 
shown by Baer:}:, commences in the extension in opposite directions of at first a faint 
indentation in the margin of the central canal, which quickly becomes deeper, and is 
carried across the surface of the yelk, and gradually more and more deepening and 
widening as it proceeds, is carried round the sides, and meeting in the middle of the 
depression on the under surface, or of the remains of the white patch when this has 
not already disappeared, is completed by passing through the middle of the yelk ; 
which is thus divided into two portions. This first division occupies from twenty to 
thirty minutes before it is finished, and it is not until then that a second fissure is 
commenced. I have not had any opportunity of proving whether this division is the 
direct result of subdivision of the central vesicle, and the attraction of the yelk-cells 
in equal proportions around each division of that body, as believed by K6lliker§, 
and, as it seems fair to infer, is the case ; but in addition to the observation by Prof. 
Sharpey||, that the contraction of the entire yelk at the commencement of these 
changes, and the movements he has observed among its granules as they proceed, are 
in favour of this opinion — I may remark, that the extension of the yelk of the Frog’s 
* Loc. cit. tom. ii. p. 104, 1824. f Muller’s Archiv, 1834. J Loc. cit. 
§ Muller’s Archiv, 1843. || Quain’s Anat., Fifth Edition, 1848. 
2 B 2 
