74 



OBSTETRICAL PHYSIOLOGY. 



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 intra-fcetal portion ; and the 

 livihilical vesicle, vesicula alba, or extra-fcetal portion. These two 

 divisions communicate by the omplialo-mescnteric 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 enlargement, which the intestinal furrow pushes 

 towards the inferior part of the abdominal cavity ; the enlargement 

 becomes elongated and vesicular-looking, and gradually increases in 

 size by bringing the umbilical vessels towards its borders. Becoming 



Fig. 44. 



Ovum about a Month Ou>. 



1, Vitelline Membrane or Primitive Chorion, which has almost disappeared ; 

 •2, External Layer of the Blastoderm or Second Chorion ; 3, Allantois pene- 

 trating the Villosities ; 4, Umbilical Vesicle ; 5, Union of the Caudal and 

 Cephalic Processes, and Formation of the Cavity of the Amnion ; 6, Embryo ; 

 7, Allantois. 



still more elongated, it passes through the umbilicus and spreads itself 

 over the inner face of the chorion, betw^een the latter and the amnion. 

 It is divided into two sacs by a constriction, the urachiis, at the 

 umbilical ring ; the inner sac is the smallest, and forms the bladder ; 

 while the external, the most voluminous, composes the ^jro^er 

 allantois. 



We have now seen that the vitelline membrane, lined by the serous 

 vesicle, forms a complete shell around the ovum in process of develop- 

 ment ; and that the layers of the blastoderm eventually constitute three 

 membranous sacs, two of which envelop the embryo. These various 

 membranes constitute the " envelopes '' or " annexes " of the young 

 creature during utero-gestation, and in describing them it is usual to 

 include the " cord" composed of bloodvessels, as well as the capillary 

 ramifications which establish and maintain such important relations 

 between the mother and its offspring during this period. 



