THE FCETUS. 



899 



that of the principal hippomanes, and if pressed between tlie fingers, the 

 brown matter contained in a thin- walled sac escaped by the pedi'cle and 

 spread itself over the external surface of the chorion. There the villosities 

 ot the placenta were absent at the margin of the opening, which was 

 surrounded by a whitish areola (Fig. 425, h). 



"Might it not be admitted, from this disposition, that the hippomanes is 

 developed between the placenta and the uterus, and is carried inward, by 

 pushing before it the chorion and layer of the allantois covering it, until, 

 on reaching the allantoid cavity, it becomes detached, like certain fibrous or 

 cartilaginous bodies in the synovial or serous cavities ?" — F. Lecoq. 



4. The Umbilical Vesicle. 



The umbilical vesicle is a small fusiform or pyriform pouch, lodged in the 

 infundibulum at the extremity of the umbilical cord. Its fundus adheres to 

 the chorion ; the opposite extremity is prolonged to a variable depth in the 

 substance of the cord, and is even continued, in the very young foetus, to -the 

 abdominal cavity, by a narrow canal that communicates with the terminal 

 portion of the small intestine. 



This pouch has a red colour, from its great vascularity ; its walls 

 receiving a special artery derived from the anterior mesenteric, the 

 corresponding vein passing to the portal vein. These are the two omjihalo- 

 mesenteric vessels. 



In the last months of foetal life, the umbilical vesicle is always more or 

 less atrophied ; its cavity has disappeared, and it is no more than a thin 

 reddish-brown cord. Its vessels also become atrophied in the same manner, 

 and nearly always nothing is found but the artery reduced to the dimensions 

 of a thread. 



, 5. The Placenta. 



In Solipeds, the placenta is composed of a multitude of small tubercles, 

 spread uniformly over the external 



surface of the chorion, which they ^'S- 426. 



almost completely cover. These 

 small tubercles are formed by an 

 aggregation of extremely vascular 

 villi, which implant themselves in 

 the follicles of the uterine mucous 

 membrane. The terminal ramifi- 

 cations of the vessels of the cord 

 constitute the vascular apparatus of 

 these villi (Fig. 426 ). 



Stettctueb. — The villosities of 

 the placenta are composed of a 

 small quantity of delicate nu- 

 cleated connective tissue (and a 



basement membrane), covered by simple epithelium ; at their centre, they 

 present the capillary ramifications of the vessels of the cord, which generally 

 form loops that return towards the base of the papilla (Fig. 427). 



The villous tufts penetrate the maternal uterine mucous membrane, in 

 such a manner that the two capillary systems of mother and foetus are only 

 separated by the very thin walls of the vessels and the epithelium of the 



PORTION' or THE ULTIMATE RAMIFICATIONS 

 OP THE UMBILICAL VEriSELS, FORJIIXG THE 

 FCETAL VILLI OF THE PLACliNTA. 



