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 BAER!, 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 KOLLIKER, 

 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. torn. ii. p. 104, 1824. f MULLEE'S Archiv, 1834. } Loc. tit. 



MULLER'S Archiv, 1843. || QUAIN'S Anat., Fifth Edition, 1848. 



2 B 2 



