70 



OBSTETRICAL PHYSIOLOGY. 



enlargement, which the intestinal furrow pushes towards the inferior part 

 of the abdominal cavity ; the enlargement becomes elongated and vesicu- 

 lar-looking, and gradually increases in size by bringing the umbilical 

 vessels towards its borders. Becoming still more elongated, it passes 

 through the umbilicus and spreads itself over the inner face of the 

 chorion, between the latter and the amnion. It is divided into two sacs 

 by a constriction, the urachus, at the umbilical ring ; the inner sac is the 

 smallest, and forms the bladder; while the external, the most voluminous, 

 composes the proper 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 

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

 membranes constitute the " envelopes " or " annexes " of the young crea- 

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

 the " cord " composed of blood-vessels, as well as the capillary ramifica- 

 tions which establish and maintain such important relations between the 

 mother and its offspring during this period. 



Fig- 35- 

 Chorion of the Mare at Mid-term 



Inflated. 



A, Posterior portion occupying the body of the Uterus ; B, Left Comu plicated and sacculated 

 C, Right Comu, longer than the left, and containing a portion of the Foetus. 



SECTION II. ANNEXES OF THE FCETUS. 



The annexes, then, comprise the chorion^ a membraneous envelope 

 exactly adapted to the uterus ; the amnion, a second ovoid sac included 

 within the latter, and containing the foetus ; the allantois, a membrane 

 composed of two layers, which are spread over the external face of the 

 amnion and the inner surface of the chorion, and thus lines the cavity 

 formed by these two envelopes \ 'a small bladder of a pyriform shape, the 

 umbilical vesicle ; the //«<r^«/rtr, a collection of vascular tufts grafting the 

 foetus to the mother ; and the umbilical cord, composed of vessels that 

 attach the foetus to the envelopes which contain it, and which ultimately 

 ramify in the placental tufts. We will describe each of these in detail, as a 

 correct knowledge of their anatomy is essential to the* comparative ob- 

 stetrist. As before, we will take solipeds as the type of comparison. 



