DEVELOPMENT OP THE EMBRYO. 989 



take place in the yelk, the whole sphere of which is first marked out by a fur- 

 row into two hemispheres, and is at last completely divided by the extension of 

 this fission to the centre ; each half is again furrowed and then cleft in the same 

 manner, and thus the entire yelk is broken up into a mass of segments (Fig. 

 261, A, B, c). This " segmentation" takes place pari passu with the multipli- 

 cation of the embryo-cells, each of which is surrounded by a distinct portion of 

 the yelk ; and there seems every probability that it is determined by that mul- 

 tiplication, and that each cell of the pair that is formed by the duplicative sub- 

 division of its predecessor draws around itself its proper share of the nutritive 

 material. These changes take place in the Mammalian Ovum, during its 

 transit along the Fallopian tube to the uterus ; so that, by the time of its arrival 

 there, the whole cavity of the Zona pellucida is occupied by minute spherules 

 of yelk, each containing a transparent vesicle, 1 the aggregation of which gives 

 it a mulberry-like aspect (Fig. 261, D) ; and by a continuance of the same pro- 

 cess o"f subdivision, the component segments becoming more and more minute, 

 the mass comes to present a finely granular aspect. 



996. At this stage it does not appear that the several segments of the yelk 

 have a distinct enveloping membrane ; but an envelop is now formed around 

 each of them, converting it into a cell, of which the included vesicle constitutes 

 the nucleus, and of which the portion of the yelk surrounding this forms the 

 contents. This happens first to the peripheral portions of the mass ; and as its 

 cells are fully developed, they arrange themselves at the surface of the yelk 

 into a kind of membrane, and at the same time assume a pentagonal or hexa- 

 gonal shape from mutual pressure, so as to resemble pavement-epithelium 

 (Plate I. Fig 5). As the globular masses of the interior are gradually con- 

 verted into cells, they also pass 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 yelk remains filled only with a clear 

 fluid. By this means the exterior of the yelk is speedily converted into a kind 

 of secondary vesicle situated within the Zona pellucida, and named by Bischoff 

 the Uastodermic vesicle. This vesicle, very soon after its formation, presents at 

 one point an opaque, roundish spot (Plate I. Fig. 6), which is produced by an 

 accumulation of cells and nuclei of less transparency than elsewhere ; this is 

 termed the area germinativa. The wall of the vesicle, which is termed the 

 germinal membrane, increases in extent and thickness by the formation of new 

 cells (whose mode of production has not been clearly made out) , and it sub- 

 divides into two layers (Plate I. Fig. 7), which, although both at first composed 

 of cells, soon present distinctive characters, and are concerned in very different 

 ulterior operations. The outer one of these is commonly known as the serous 

 layer (Fig. 8) ; but being the one in whose substance the foundation is laid for 

 the vertebral column and the nervous system, it is sometimes called the animal 

 layer. The inner one is usually known as the mucous layer (Fig. 9) ; and 

 being the one chiefly concerned in the formation of the nutritive apparatus, it 

 is sometimes called the vegetative layer. This division is at first most evident 

 in the neighborhood of the area germinativa ; but it soon extends from this 

 point, and implicates nearly the whole of the germinal membrane. 



997. The Area Germinativa, at its first appearance, has a rounded form; but 

 it soon loses this, becoming first oval, and then pear-shaped (Plate II. Fig. 11). 

 While this change is taking place in it, there gradually appears in its centre a 

 clear space, termed the area pellucida (a) ; and this is bounded externally by a 



1 This vesicle has not yet been made out, in the Mammalian ovum, to be a true cell, 

 which it certainly is in the ovum of many of the lower animals ; its appearance, when 

 liberated from the yelk- granules which surround it, being rather that of a fat or oil- 

 globule. 



