258 



EMBRYOLOGY 



MATERNAL BLOOD VESSEL 



INTERCHANGE 

 ACROSS 

 CHORION 



EMBRYONIC 

 BLOOD VESSEL 



FROM EMBRYO 



C02 



UREA 



URIC ACID 



ETC. 



Fig. 168. The placental barrier 

 in the human. The surface of the 

 villus is bathed by maternal blood. 

 Materials passing from maternal 

 blood to fetal blood go through the 

 chorion, which is composed of two 

 layers of cells, and the endothelium 

 of the fetal blood capillaries. Three 

 membranes thus separate the two 

 blood streams. 



THE PLACENTAL BARRIER 



prevented in order that the embryo may remain attached to the uterine wall 

 during development. At the same time the rhythmic changes in the ovary 

 must be stopped so that no ovulation occurs while the egg is developing. 

 Finally, at birth the mammary glands must be ready to secrete milk. 



These rhythmic processes are controlled through the production of hor- 

 mones by the pituitary, the ovary, and the placenta. We shall begin by out- 

 lining the rhythmic changes which occur in the ovary. The discussion which 

 follows deals mainly with the human, although most of the experiments are 

 carried out on other mammals. 



The ovarian cycle 



The ovarian egg develops in a follicle which increases in size and the egg 

 remains attached to the inner wall of the follicle (Fig. 169) . When ovulation 

 takes place the wall of the follicle breaks, the follicle collapses, and the egg is 

 liberated, together with a layer of cells. The blood vessels break, and a blood 

 clot fills the cavity of the follicle. Then this follicle begins to be filled with 

 cells, forming a solid body, the corpus luteum. This corpus luteum persists for 

 some time and then degenerates into scar tissue. A new follicle completes its 

 growth cycle, and this cycle repeats itself until fertilization takes place (see 

 Chap. 2). 



There are two important parts to this cycle. One is the growth of the 

 follicle; the other is the growth of the corpus luteum. The changes in the 

 ovary are paralleled by changes in the walls of the uterus. The diagram of the 

 uterus in Figure 169, shows the opening of the oviducts into the uterus and 

 the walls of the vagina. The uterine wall is rather thick and undergoes 

 periodic changes, which are correlated with the ovarian cycle. 



