498 



HISTOLOGY 



uterine layer degenerates, or it may already be broken up by the men- 

 strual process. It is destroyed and removed until only the lower parts 



of the gland are left, to- 

 gether with a residual con- 

 nective-tissue layer about 

 one third as thick as the 

 original membrane. 



The distal ends of the 

 uterine blood vessels are 

 also lost in this process, 

 and the maternal blood 

 comes out of the free ends 

 of the arteries and circu- 

 lates in the open spaces 

 that lie between the uterus 

 and chorion, and among 

 the villi. As the broken 

 ends of the uterine veins 

 also remain open, this blood 

 is returned through them 



FIG. 467. Diagram of the relations of the foetal mem- 

 branes to the uterus in man at the close of pregnancy. 

 (After SCHAPER.) 



to the maternal circula- 

 tion. Figure 467 is a dia- 

 gram after Schaper to illustrate this condition. 



During this development the simple epithelium on the villi has pro- 

 liferated an outer layer, which differs in appearance from the original 

 layer, which now lies in a basal portion on the connective tissue. 

 This second and distal layer is syncytial, in that its cell boundaries are not 

 demonstrable. It is thickest on the tips of the villi, and is incomplete 

 nearer the chorion. Where it is thickest, it is developed into tuberosi- 

 ties much like those on the umbilical cord. When this membrane is 

 torn away from the uterine wall, the blood vessels close and establish new 

 connections between arteries and veins, the reticular tissue is thick- 

 ened, and a new covering of epithelium is regenerated from the re- 

 maining portions of the glands. 



It should be noticed, in the apparatus described above, that the food 

 materials and other materials are exchanged directly from parental 

 blood to embryonic blood, and vice versa through only a few thin cells. 

 In the elasmobranch fishes, on the other hand, a uterine fluid stood as 

 an additional agent of transfer. Also some or all of the food material 

 probably entered the young fish's body through the digestive tract in- 

 stead of directly into the blood. 



The work of nourishing the offspring from the parental body is not 

 finished even with their birth, in the mammals. It is then taken up by 



