DEVELOPMENT. 7(;9 



The cause of this singular subdivision of the yolk is quite obscure : 

 though the immediate agent in its production seems to be the central 

 vesicle contained in each division of the yolk. Originally there was prob- 

 ably but one vesicle, situated in the centre of the entire granular mass 

 of the yolk, and probably derived in the manner already described from 

 the germinal vesicle. This divides and subdivides : each successive divi- 

 sion and subdivision of the vesicle being accompanied by a corresponding 

 division of the yolk. 



About the time at which the mammalian ovum reaches the uterus, 

 the process of division and subdivision of the yolk 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, ap- 

 pears 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 yolk 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 

 spheroidal masses, each of which contains a clear vesicle or nucleus in 

 its centre, and is, in fact, an embryonal cell. The zona pellucida, and 

 the layers of albuminious 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 yolk into a kind of 

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

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

 interior pass gradually to the surface and accumulate there, thus in- 

 creasing the thickness of the membrane already formed by the more 

 superficial layer of cells, while the central part of the yolk remains filled 

 only with a clear fluid. By this means the yolk is shortly converted 

 into a kind of secondary vesicle, the walls of which are composed exter- 

 ntilly of the original vitelline membrane, and within by the newly formed 

 cellular layer, the blastoderm or germinal membrane, as it is called. 



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

 tion of what is known as incomplete or partial segmentation, or mero- 

 blastic segmentation. In the youngest ova the germinal vesicle is situ- 

 ated subcentrally, but as development proceeds it passes to the periphery, 

 and the protoplasm surrounding it remaining free from yolk granules, 

 the germinal disc is formed. This germinal disc is not marked out by 

 any sharp line from the remaining protoplasm, but passes insensibly 

 into it. The first change consists in the appearance of a furrow run- 

 49 



