CLEAVAGE OF THE YELK. 741 



quite obscure : though the immediate agent in its produc- 

 tion seems to be the central vesicle contained in each 

 division of the yelk. Originally there was probably but 

 one vesicle, situated in the centre of the entire granular 

 mass of the yelk, and probably derived from the germinal 

 vesicle. This, by some process of multiplication, divides 

 and subdivides : then each division and subdivision attracts 

 around itself, as a centre, a certain portion of the sub- 

 stance of the yelk. 



About the time at which the mammiferous ovum reaches 

 the uterus, the process of division and subdivision of the 

 yelk appears to have ceased, its substance having been 

 resolved into its ultimate and smallest divisions, while its 

 surface presents a uniform finely-granular aspect, instead 

 of its late mulberry-like appearance. The ovum, indeed, 

 appears at first sight to have lost all trace of the cleaving 

 process, and, with the exception of being paler and more 

 translucent, almost exactly resembles the ovarian ovum, 

 its yelk consisting apparently of a confused mass of finely 

 granular substance. But on a more careful examination, 

 it is found that these granules are aggregated into 

 numerous minute spherical masses, each of which contains 

 a clear vesicle in its centre, but is not, at this period, 

 provided with an enveloping membrane, and possesses 

 none of the other characters of a cell. The zona pellucida, 

 and the layer of albuminous matter surrounding it, have 

 at this time the same character as when at the lower part 

 of the Fallopian tube. 



The time occupied in the passage of the ovum, from the 

 ovary to the uterus, occupies probably eight or ten days in 

 the human female. 



Shortly after this, important changes ensue. Each of 

 the several globular segments of the yelk becomes sur- 

 rounded by a membrane, and is thus converted into a cell, 

 the nucleus of which is formed by the central vesicle, the 

 contents by the granular matter originally composing the 



