284 Veterinary Obstetrics 



At this stage the external layer, consisting of the smaller cells, 

 and the internal mass of larger ones are firmly attached to each 

 other at one point only. The segmented ovum is still surrounded 

 by the vitelline membrane, the zona radiata and the layer of al- 

 bumen which it had acquired during its progress through the 

 oviduct. 



Within a few hours after entering the uterus the ovum has be- 

 come greatly enlarged, owing to the accumulation of a fluid be- 

 tween the external layer of small cells and the mass of larger in- 

 ner cells, except at the point of attachment between the two, so 

 that the inner mass occupies a comparatively small area at the 

 superior pole of the ovum. This stage in the development of 

 the ovum is known as the blastodermic vesicle. The vesicle con- 

 sists of an outer wall of flattened, polygonal cells, which have 

 been formed from the outer cells of the previous stages, to which 

 is attached, at one of its poles, the small mass of large cells. 

 Separating the two layers of cells, except at the polar point of at- 

 tachment, is a relatively large quantity of fluid, which greatly dis- 

 tends and increases the size of the vesicle. The mass of inner 

 cells is flattened out in the form of a disk, consisting of several 

 layers of somewhat spherical cells at the center, which is known 

 as the embryonal or germinal area, while, at the pheriphery, 

 they are but one or two layers in thickness. As the vesicle in- 

 creases rapidly in size the vitelline membrane disappears, while 

 the zona radiata and the albuminous layer become greatly at- 

 tenuated by being stretched out. 



During this stage of development, which continues in the rab- 

 bit to about the seventh day, the ovum lies free within the uter- 

 ine cavity and, in case of multiparous animals, the ova, which 

 enter the uterus almost simultaneously, tend to distribute them- 

 selves at more or less uniform distances apart throughout the or- 

 gan and assume the positions which they are to retain through- 

 out their period of intra-uterine development, the location of 

 each being early indicated b}' a bulging of the uterine walls. 



Germinal IvAyers. 



Toward the close of the developement of the blastoderm, im- 

 portant changes take place, by which there are established three 

 distinct germinal layers, each of which is destined to take a spe- 



