CHANGES IN THE OVUM. 



69 



over the internal face of the chorion and the surface of the embryo with- 

 out any limit. But the inflection or bending of the latter at its two 

 extremities causes the external layer to become depressed, and to consti- 

 tute two folds ; one investing the head, the other the opposite extremity 

 of the body. The lateral parts of the embryo are also inflected towards 

 each other to form the thoracic-abdominalcavity, and it is thus enclosed 

 in the serous layer, whose sides soon meet above the back of the young 

 creature, and a short pedicle — the superior umbilicus — ^joins the two 

 portions of the blastoderm This pedicle soon disappears, and the 

 embryo is then inclosed in an independent sac — the amnion already 

 alluded to. The sensitive layer is, therefore, decomposed into two sec- 

 tions : an internal, the amnion ; and an external, the serous vesicle, which 

 is applied against the inner face of the zona pellucida, and concurs to 

 form the chorion. 



The middle layer will be noticed hereafter, and now we go to the internal 

 layer. The intra-fcetal parts of this so-called " mucous layer " form the 

 epithelium of the intestines and the glands belonging thereto, the respira- 

 tory apparatus, the kidneys, and the bladder. These organs are developed 

 it the same time as the extra-foetal parts. 



Fig. 34- 



Ovum about a Month Old. 



I, Vitelline Membrane or Primitive Chorion, which has almost disappeared ; 2, External Layer 

 of the Blastoderm or Second Chorion ; 3, Allantois penetrating the Villosities ; 4, Umbilical 

 Vesicle ; 5, Union of the Caudal and Cephalic Processes, and Formation of the Cavitv of the 

 Amnion ; 6, Embryo ; 7, Allantois. 



In curving-in upon itself, the embryo encloses a portion of the internal 

 blastodermal layer, but the union between the free and the imprisoned 

 parts is at first largely maintained ; soon, however, it contracts, and in a 

 brief period is only represented by a somewhat narrow canal, the inferior 

 umbilical ring. The mucous layer is therefore divided into two distinct 

 portions : the intestinal furrow or intrafoetal portion ; and the umbilical 

 vesicle, vesicula alba, or extra foetal portion. These two divisions com- 

 municate by the omp halo -mesenteric or vitelline duct. The " umbilical 

 vesicle " is filled with a granular fluid, which is conveyed by the omphalo- 

 mesenteric vessels for the nutrition of the foetus ; when this alimentary 

 reserve is nearly expended, the allantois appears. This begins by a small 



