MAMMALIAN EMBRYOLOGY 627 



But as the student must know the placental animals in order to make 

 the most of his study in Comparative Anatomy, it is essential that he at 

 least obtain a clear and accurate understanding of the two principal 

 types of placental formation. 



In the first place, the placenta may be defined as consisting of all 

 structures affecting nutritive, respiratory, and excretory interchanges 

 between the embryo and its mother in viviparous animals. It is evident, 

 then, that the placenta must form in the region where the trophoblast 

 comes in contact with the uterine mucosa, and that the trophoderm 

 itself plus the vascularization in the yolk-sac, allantois, or chorion, will 

 be the elements from which the placenta is developed. (Figs. 358, 359.) 



At this stage the student must review the chapter on the develop- 

 ment of the extra-embryonic membranes in the chick. 



The rabbit is often used as an example of a form of mammalian em- 

 bryology which can be contrasted with the embryological development 

 of the chick. In the rabbit the extra-embryonic membranes develop quite 

 like those in the chick, except that the point of fusion of these mem- 

 branes consists of only a small knot, whereas in the chick this fusion 

 takes the form of an elongated seam. In both rabbit and chick the tail- 

 fold grows more rapidly than the head-fold. 



In man, where entypy takes place (that is, where the trophoblast 

 remains continuous about the entire blastoderm), the extra-embryonic 

 membranes do not grow as in the rabbit and chick, but by a splitting of 

 the ectoderm to form the beginning of the amniotic cavity. 



The forming of the extra-embryonic membranes in man quite natur- 

 ally causes the embryo to remain connected with the blastodermic wall 

 by a body-stalk (Fig. 358, B). The separating of the ectoderm imme- 

 diately above the embryo to form the amniotic cavity causes the embryo 

 to form the floor of this cavity, while the trophoblast forms the roof. 

 The sides, or walls, of the cavity meet the embryo at the edges of the 

 embryonic shield. 



But, whether the amniotic cavity is formed as in the rabbit or as in 

 man, the walls of the cavity extend ventrally until they surround the 

 umbilicus. 



The yolk-sac and the yolk-stalk, as well as the allantois, although 

 quite small in man, are pushed into this body-stalk or umbilical-stalk. 

 The amniotic cavity grows large in man and contains from one-half to 

 one liter of liquor amnii. 



THE YOLK-SAC 



The open space on the interior of the mammalian blastodermic vesi- 

 cle is supposed to represent the yolk-sac (Fig. 359, G) of such animals 

 as the chick and the frog; and, as this open space is relatively very large, 

 the yolk-sac occupies the main portion of the early mammalian blasto- 

 dermic vesicle. The cavity of the vesicle opens into the mid-gut region 



