70 



OBSTMTHmAL PMrSI^M^iV, 



is now seen to bo occupied by a large vesicle, coafeainiug a fluid and 

 dark granules. In the centre of this fluid is a spherical body, composed 

 of a substance having a finely granular appearance, and containing a 

 cavity flUed with a colourless and pellucid fluid; this hollow and 

 spherical body seems to be the true germ. The vesicle oontaining 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 pres- 

 sure from within flattens these corpuscles, until they resemble pavement 

 epithelium : and, finally, they all coalesce to compose a membrane 

 lining the zona, which has been named the blastoderm. This blasto- 

 dermic 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. 



Fig. 40. 

 Ovxm v«om the UTEBrerii: ealji' ov 



OrtDUGT. 



r ig. 41. 



OvTM ritOM THE Ul'EEIXE BSD Off THE 

 OvLUrCT, WITH A>" AUPmOBtil 

 LATER OF ALBCMtS. 



Though the foregoing changes in the impregnated ovum have been 

 chiefly observed in the Rabbit and Guiuea-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 utems ; 

 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 proce^ 

 the zona thins away and fixially disappears, and a mass of albuminoiel 

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

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

 hoEow sphere, the hollow wall of which ofl:ers two layers : both consist- 

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

 and proportion of the oil-globules. 



