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HAND-BOOK OF PHYSIOLOGY. 



of the yelk, and probably derived from the germinal vesicle. This divides 

 and subdivides: each successive division and subdivision of the vesicle 

 being accompanied by a corresponding division of the yelk. 



About the time at which the Mamma- 

 lian 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 divi- 

 sions, while its surface presents a uniform 

 finely-granular aspect, instead of its late 

 mulberry-like appearance. The ovum, in- 

 deed, appears at first sight to have lost all 

 trace of the cleaving process, and, with the 

 exception of being paler and more trans- 

 lucent, almost exactly resembles the ova- 

 rian ovum, its yelk consisting apparently of 

 a confused mass of finely granular sub- 

 stance. But on a more careful examina- 

 tion, it is found that these granules are 

 aggregated into numerous minute sphe- 

 roidal masses, each of which contains a clear 

 vesicle or nucleus in its centre, and is, in 

 fact, an "embryonal cell." The zona pel- 

 lucid a, 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 passage of the ovum, from the ovary 

 to the uterus, occupies probably eight or 

 ten days in the human female. 



When the peripheral cells, which are 

 formed first, are fully developed, they 

 arrange themselves at the surface of the 

 yelk into a kind of membrane, and at the 

 same time assume a polyhedral shape from 

 mutual pressure, so as to resemble pave- 

 ment epithelium. The deeper cells of the 

 interior pass gradually to the surface and 

 accumulate there, thus increasing the thick- 

 ness of the membrane already formed by the more superficial layer of 

 cells, while the central part of the yelk remains filled only with a clear 

 fluid. By this means the yelk is shortly converted into a kind of secondary 

 vesicle, the walls of which are composed externally of the original vitelline 

 membrane, and within by the newly formed cellular layer, the Uastodermic 

 or germinal membrane, as it is called. 



FIG. 406*. Diagrams of the various 

 stages of cleavage of the yelk (Dalton). 



