408 VERTEBRATE ZOOLOGY 



blast, is distinguished from the inner-cell-mass. Subsequently (Fig. 

 213, B and C) the trophoblast separates from the inner-cell-mass 

 except at the animal pole and a large cavity filled with fluid appears 

 between the two layers. The trophoblast layer is a temporary struc- 

 ture serving as a sort of primitive placenta for the young embryo 

 and helping the latter to gain its first connection with the uterine 

 membrane. A speciaUzed region of the trophoblast, called the 

 Trager, sends short papillae into the uterine mucosa, opening the 

 way for the true placental villi that come later. The inner-cell-mass 

 forms the entire embryo, together with the embryonic membranes, 



amnion, chorion, allantois, and 

 yolk-sac. At first a round ball 

 of cells, the inner-cell-mass flat- 

 tens out to form a thin lens- 

 shaped mass in contact with the 

 attached part of the trophoblast, 

 or Trager. Later two layers form, 

 ectoderm and endoderm, by a 

 sorting out of two types of cells, 

 or a migration inwards of the 

 , ,^^ ^^r, endoderm cells. This process is 



^^^S^a^,/^^^^^ t^® equivalent of the first step in 



Fig. 214.-Section through the fully gastrulation, but cannot readily 

 formed blastodermic vesicle of the rab- be compared with the equivalent 

 bit./cm, granular cells of inner cell mass; process in any other type of em- 

 iroT>h, trophoblast; z. p, zona pellucida , ^ i.u j. i j 



(Fro^Kellicott, after Quain.) ^ryo. Once the two-layered germ- 



inal disk, early gastrula, is formed, 

 the remainder of the process of embryogenesis is much like that of 

 the Sauropsida and need not be further described. 



The development of the embryonic membranes, however, differs 

 in many ways from that seen in the bird. The layer of endoderm, 

 at first confined to the upper part of the vesicle, spreads until it forms 

 a complete inner lining for the trophoblast. The gut of the embryo 

 is pinched off from the upper part, leaving an empty yolk-sac below, 

 connected with the gut-endoderm by a slender yolk-stalk. The 

 amnion sometimes forms as in the chick (Fig. 216), by a fold of the 

 somatopleure, which also produces the outer layer or chorion; but 

 sometimes the amnion forms by means of a cavity opening up in 

 the midst of the ectodermic mass, a short-cut method used by the 



