580 



GENERATION AND DEVELOPMENT. 



FIG. 215. 



sents a imiform finely-granular aspect, instead of its late mul- 

 berry-like appearance. The ovum, indeed, appears at fii\<t 



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 ap- 

 parently of a confused mass 

 of finely granular substance. 

 But on a more careful exam- 

 ination, 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, pro- 

 vided with an enveloping mem- 

 brane, 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 surrounded 

 by a membrane, and is thus 

 converted into a cell, the 

 nucleus of which is formed by 

 the central vesicle, the con- 

 tents by the granular matter 

 originally composing the glob- 

 ule: these granules usually 

 arrange themselves concen- 

 trically around the nucleus. 



Diagrams of the various stages of cleav- When the peripheral Cells, 



age of the yelk (after Daiton). which are formed first, are 



