8 14 EMBRYOLOGY 



number of granules, and colossal cells with enlarged and multiple nuclei. 

 A portion of this membrane is not thrown off with the placenta in par- 

 turition, but processes extend into the placenta and closely surround 

 the foetal tufts. 



The two arteries of the umbilical cord branch on the foetal surface 

 beneath the amnion and finally penetrate the substance of the placenta. 

 The branches of the veins, which are about sixteen in number, converge 

 toward the cord and unite to form the umbilical vein. On the uterine 

 surface of the placenta are oblique openings of a large number of veins 

 which return the maternal blood to the uterine sinuses. There are, also, 

 the small spiral arteries, which pass into the substance of the placenta 

 to supply blood to the maternal portion. These are the " curling 

 arteries," described by John Hunter. If the umbilical arteries are 

 injected, the liquid is returned by the umbilical vein, having passed 

 through the vascular tufts of the foetal portion of the placenta. 



According to Winkler, there are three kinds of foetal villi : i. Those 

 which terminate just beneath the chorion, without penetrating the vascu- 

 lar lacunae. 2. Longer villi, which hang free in the lacunae. 3. Long, 

 branching villi, which penetrate more deeply into the placenta, some 

 extending as far as its uterine surface. 



The great vascular spaces, or lacunae of the maternal portion of the 

 placenta, present a number of trabeculae, which extend from the uterine 

 to the foetal surface ; and between these trabeculae, are exceedingly 

 delicate transverse and oblique secondary trabecular processes. The 

 bloodvessels of the foetal tufts are surrounded with a gelatinous con- 

 nective-tissue structure, and as late as the sixth month are covered with 

 a layer of chorionic cells. 



The mode of formation of the vascular spaces in the placenta has 

 been a subject of much discussion. The following, however, seems to 

 be the most reasonable view in regard to this question : That portion of 

 the uterine mucous membrane which becomes the maternal portion of 

 the placenta extends from the decidua serotina and surrounds the villi, 

 which are embedded in its substance. As the arborescent villi extend, 

 they encroach on the bloodvessels of the prolongations from the 

 serotina, which latter become much enlarged and finally form the great 

 vascular spaces traversed by the trabeculae mentioned above. At term, 

 however, according to Heinz (1888), the foetal vessels have lost their 

 covering of epithelium that is observed in the earlier months of preg- 

 nancy. Thus the most important parts of the placenta are formed by an 

 interlacement of the villi of the chorion with the altered structures of the 

 mucous membrane of the uterus. 



In the human subject the maternal and foetal portions of the pla- 



