mesoderm 



trophoderm 



amnion (and mesoderm) 



trophoderm and mesoderm (chorion) 

 yolk sac (and mesoderm) 



vitelline vein 



extraembryonic coelom 



vitelline artery 



terminal sinus (arterial) marks 

 limit of mesoderm 



left allantoic artery 

 left allantoic vein 

 mesoderm free yolk sac 



Figure 7-18. Yolk-sac placenta, enclosing allantoic vesicle, of the opossum. A, and details of the 

 membranes of the placenta, B. (After de Lange) 



The vitelline arteries unite and the single vessel passes 

 down the posterior surface of the yolk-sac neck, then below 

 the allantoic sac, and up behind that sac and back to the 

 terminal, arterial sinus. The terminal sinus encircles the 

 yolk mass at its middle. From here the blood is distributed 

 to the ventral half of the yolk sac as well as the dorsal part. 

 The yolk sac below the sinus terminalis has placental villi 

 which interdigitate with processes of the lining of the mater- 

 nal uterus, at least in the opossum. 



The amount of nutritive material exchanged is probably 

 slight. The young in the opossum are born about three days 

 after the development of the yolk-sac placenta. In some 

 marsupials the yolk-sac placenta is better developed and 

 the period of its utilization is increased. In at least two 

 genera, Parameles and Phascolarctus, the allantois reaches the 

 chorion and fuses with its inner surface, so that during the 

 later stages of placentation both vitelline and allantoic 

 (chorio-allantoic) placentas function side by side. 



In the placental mammals, one sees the gradual decrease 

 in the importance of the yolk-sac placenta and an increase 

 in the allantoic placenta. In this group, the yolk sac is 

 never very large, nor does it form a part of the placenta 

 (that part of the serosa which develops finger-like extensions 

 or villi and attaches to the uterine wall). The allantois 

 changes as its placenta becomes more efTective. It changes 

 from a large vesicle to a rather rudimentary one, but its 



blood vessels supply the mesoderm of the chorion and thus 

 the placenta. 



To increase the effectiveness of the placenta, it is neces- 

 sary that the amount of tissue between the embryo and the 

 parent be reduced. Various grades of reduction occur. In 

 the epitheliochorial type, the embryo and maternal tissue 

 lie in close interdigitating contact (pig — Figure 7-19). In the 

 syndesmochorial type, the maternal uterine epithelium is 

 eroded away between the embryonic membranes and the 

 connective tissue of the uterus. The endotheliochorial tyj)e 

 goes further with the erosion of the connective tissue, so 

 that the embryonic tissues contact the maternal blood ves- 

 sels (cow, sheep, goat). In the most advanced type, the 

 hemochorial type, the embryo's blood vessels, enclosed by 

 connective tissue, are bathed in the maternal blood. 



The types of placentation observed in mammals are sev- 

 eral (Figure 7-20): diffuse, having villi over the whole sur- 

 face of the extraembryonic membranes (pig); cotyledonar>', 

 having patches of placental villi over the entire surface of 

 the extraembryonic membranes (cow); zonory, having a 

 band of villi encircling the membranous cover (cornivores); 

 and discoidal, in which there is a single round area of villi 

 (man) or two such areas (monkey). Generally, the embryo 

 within its membranes lies in the uterine cavity, but in man 

 it develops within the tissues of the uterine wall. 



It is difficult to say whether there is any comparative 



212 



EMBRYOGENESIS OF THE CHORDATES 



