914 CARE AND NOURISHMENT OF THE DEVELOPING YOUNG 



forming allantoic diverticulum with the chorionic mesoderm is called the 

 body stalk (fig. 372B). 



3. The Reproductive Duct as a Protective Embryonic 



Membrane 



The developing egg is retained within the oviduct in all metatherian and 

 eutherian mammals and in various species in the other major vertebrate 

 groups with the exception of the birds. Even in the birds (fig. 157), a partial 

 development of the egg normally occurs within the confines of the oviduct. 

 Oviparity thus encroaches upon ovoviviparity in birds, and ovoviviparity in- 

 fringes upon viviparity in certain sharks (Squalus acanthias), reptiles (vari- 

 ous snakes and lizards), and prototherian mammals. However, oviparity has 

 this feature which distinguishes it from ovoviviparity and viviparity, namely, 

 the new individual always hatches or leaves the confinement of the egg mem- 

 branes outside the protective environment of the reproductive duct (or other 

 protective structures). On the other hand, ovoviviparous and viviparous forms 

 are released from the egg membranes and thus "hatch out" within the oviduct 

 or other covering structure. The more viviparous the particular species, the 

 sooner the new individual hatches from its egg membranes. In most cases 

 of ovoviviparity and viviparity, the reproductive duct (specifically, the uterine 

 segment) acts as a protective embryonic membrane which surrounds the de- 

 veloping embryo or embryos. Thus, a definite area of the reproductive duct 

 is temporarily allotted to the embryo. If several embryos are present, a par- 

 ticular segment of the uterus is assigned to the care and protection of each 

 embryo (see TeWinkle, '41, '43, and '50) (fig. 371 A). For further description 

 of the uterine portion of the oviduct as a protective mechanism see p. 919. 



Fig. 373 — Continued 

 placenta is of the epitheliochorial variety, i.e., the epithelium of the chorionic tissue comes 

 into contact with the epithelium of the uterus without erosion of either. (C) Placental 

 relationships in the dog. This figure represents a small area at the edge of the zonary pla- 

 centa shown in fig. 378D, as indicated. (Redrawn and modified from Mossman, 1937, 

 Carnegie Institute Publications, vol. 26, Contributions to Embryology, No. 158.) This 

 placenta is a dual type, in that the edge of the placenta resembles somewhat the henw- 

 chorial type, i.e., maternal blood in direct contact with the chorionic epithelium of the 

 villus, while the center of the placental zone is of the endotheliochorial type of placenta- 

 tion, i.e., the epithelium of the chorionic villus is in contact with the endothelial lining of 

 the maternal blood capillaries. (D) Placental relationship in human. (Redrawn and 

 modified from Spanner, Zeitschrift fiir Anatomic, vol. 105, Julius Springer, Berlin, Ger- 

 many.) The placenta is made up of many cotyledons, each cotyledon being composed of a 

 main stem villus, which contains the larger fetal blood vessels, and from the large stem 

 villus smaller branching villi extend out into the surrounding maternal blood. Imperfectly 

 developed septa separate the various cotyledons. This type of placentation is of the hemo- 

 chorial variety, i.e. the chorionic epithelium is in contact with the maternal blood. (E) 

 Diagram illustrating the hemoendotlielial type of placentation in the late gestation period 

 of the rabbit. Here the chorionic epithelium is eroded and the capillaries of the chorionic 

 villi lie within the maternal blood. 



