66 OBSTETRICAL PHYSIOLOGY. 



spherical body seems to be the true germ. The vesicle containing it 

 disappears, and in its place is seen an elliptical depression, filled with a 

 clear fluid, and in the centre of this is the "germ," still presenting the 

 appearance of a hollow sphere. 



The fluid presses the nucleate corpuscles of the yolk outwards against 

 the inner face of the enveloping membrane, and as it increases the 

 pressure from within flattens these corpuscles, until they resemble 

 pavement epithelium ; and, finally, they all coalesce to compose a mem- 

 brane lining the zona, which has been named the blastoderm. This 

 blastodermic vesicle divides into two layers — an external and internal — 

 the first of which is pale and only slightly granular, while the cells of the 

 second are filled with fat granules; it is consequently of a deeper tint. 



Though the foregoing changes in the impregnated ovum have been 

 chiefly observed in the Rabbit and Guinea-pig, yet there can be little 

 doubt that they are of the same character in the domestic creatures 

 whose embryology we are now studying. 



It is only to be remarked that, in hoofed animals, no envelope of the 

 ovum is superadded to the zona pellucida before it enters the uterus ; 

 impregnation of the ovum taking place in the Fallopian tube, where it 

 meets the spermatozoa, the first stages of cleavage in its interior go on 

 there, but the germ-mass is completed in the uterus. In this process 

 the zona thins away and finally disappears, and a mass of albuminoid 

 matter accumulates around the ovum, which affords material for imbibi- 

 tion. The germ-mass becomes fluid at the centre, and expands into a 

 hollow sphere, whose wall offers two layers : both consisting of coherent 

 cells, and only differing, as just remarked, in the size and proportion of 

 the oil-globules. 



H 



Fig. 32- 



Blastoderm and Primitive Trace- 



a. Vitelline Membrane with its commencing Villosities ; b, External (or Serous) Layer of the 

 Blastoderm; c, Internal (or Mucous) Layer; d, Body of the Embryo; ^i, bi. Earliest 

 Cephalic and Caudal Elevation of the External Layer. 



SECTION I. DEVELOPMENT OF THE EMBRYO. 



The ovum having been lodged in the uterus, and the germ-membrane or 

 blastoderm having divided into two layers — an upper or serous^ and a lower 

 or mucous — and between which, at a later period, a vascular layer is de- 

 veloped, another modification occurs by which the outline of the embryo 

 becomes evident. 



