Reproduction 293 



work of fine blood-vessels which are connected by a pair of allantoic 

 arteries and a corresponding pair of veins to the main blood-vessels 

 of the embryo. 



In reptile and bird, the expanding allantois pushes aside such albu- 

 men as may be present, permitting the richly vascular region of the 

 allantois to come into close relation to the inner surface of the porous 

 eggshell, thus providing for exchange of respiratory gases between 

 blood and external medium. The albumen is gradually absorbed by 

 the embryonic tissues. Before the time of hatching, the shrinking yolk- 

 sac is drawn up into the growing body. The umbilical stalk — that is, 

 the whole complex of connections between the definitive body of the 

 embryo and the extra-embryonic membranes — becomes narrowly con- 

 stricted. At the time of hatching, the amnion and the slender neck of 

 the allantois are ruptured at the umbilicus. As the young animal 

 emerges, the amnion and chorion and the extra-embryonic part of the 

 allantois are abandoned. The proximal portion of the allantois, remain- 

 ing within the body, may, to a greater or lesser extent, persist as a 

 distal region of the urinary bladder of such adult reptiles as possess 

 that organ. In birds, since the adult has no urinary bladder, the proxi- 

 mal remnant of the allantois degenerates. 



Among mammals, there is some diversity as to the manner of origin 

 of the amnion and chorion. Once established, however, these mem- 

 branes possess the same relations to the germ-layers and to the defini- 

 tive body of the embryo as in reptiles. An abortive yolk-sac is present 

 but devoid of yolk except in the duckbill and spiny anteater, whose 

 maimer of development is essentially reptilian. 



The main facts concerning the development of a placenta by the 

 chorio-allantoic membrane have already been stated (see p. 242). In 

 most mammals the region of conjunction of allantois and chorion be- 

 comes thickly beset with slender, more or less branched, highly vascular 

 villi. These villi (Fig. 241) may be merely lodged in depressions in the 

 uterine wall or they may pierce more or less deeply into its tissues. In 

 extreme cases (e.g., in man) there is destruction of walls of uterine 

 blood-vessels, and the extravasated blood fills large sinuses in the 

 uterine wall. The villi project into these sinuses so that the villous 

 surfaces are directly bathed by maternal blood, an arrangement pro- 

 viding maximum efficiency in the exchange of materials between fetal 

 and maternal blood. 



Mammals exhibit various types of placenta, depending on the dis- 

 tribution of villi in the chorionic surface. When the villi are uniformly 

 distributed over the chorion, as in the horse, pig, and other ungulates, 

 the placenta is called diffuse. In most ruminant ungulates, such as 

 cattle, the villi are localized in numerous patches or clusters of varying 



