DEVELOPMENT. 659 



culty, on account of its being enveloped by the yelk-granules which 

 adhere closely to its surface. 



The cause of this singular subdivision of the yelk is quite obseure: 

 though the immediate agent in its production 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 in the manner already described 

 from the germinal vesicle. This divides and subdivides: each succes- 

 sive division and subdivision of the vesicle being accompanied by a cor- 

 responding division of the yelk. 



About the time at which the Mammalian 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, indeed, appears at 

 first sight to have lost all trace of the cleavage 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 sphe- 

 roidal masses, each of which contains a clear vesicle of nucleus in its 

 centre, and is, in fact, an embryonal 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 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 devel- 

 oped, they arrange themselves at the surface of the yelk into a kind of 

 membrane, and at the same time assume a polyhedral shape from mu- 

 tual pressure, so as to resemble pavement epithelium. The deeper cells 

 of the interior pass gradually to the surface and accumulate there, thus 

 increasing the tbickness 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 cel- 

 lular layer, the blastodermic or germinal membrane, as it is called. 



Segmentation in the Chick. The embryo chick affords an illus- 

 tration of what is known as incomplete or partial segmentation, or me- 

 roblastic segmentation. In the youngest ova the germinal vesicle is 

 situated subcentrally, but as development proceeds it passes to the peri- 

 phery, and the protoplasm surrounding it remaining free from yelk 

 granules, the germinal disc is formed. This germinal disc is not marked 

 out by any sharp line from the remaining protoplasm, but passes insen- 



