EMBRYOLOGY 



411 



Xf. 



FIG. 144. Rabbit, blastocyst. From 

 Quain. 



ect., trophoblastic ectoderm ; ent., inner cell 

 mass ; g.p., zona pellucida. 



separately, and indeed, since the implantation is intimately connected 

 with certain covering embryonic membranes that are produced in 

 the course of development, its consideration will be held over until 

 we have followed the growth 

 of similar membranes in the 

 chick. It should be borne in 

 mind, however, that this divi- 

 sion is purely artificial and 

 the changes go on side by side. 

 During the subse- 

 quent growth the internal cell 

 mass flattens and spreads out 

 until it forms a thin disc under- 

 lying the trophoblast at one 

 pole ; the mass, however, 

 soon becomes differentiated 

 intp two layers. The outer 

 layer is the ectoderm, and its 

 cells multiply rapidly and be- 

 come cubical, and in this way 

 give rise to the ectoderm of 

 the definitive embryonal shield. 



The lower layer cells form the entoderm ; they flatten out and grow 

 rapidly and in typical cases spread completely round the inside of the 

 blastocyst, which is thus made bilaminar. This, then, concludes the 

 process of gastrulation or formation of the two primary germ layers. 

 At first, in all cases, the inner cell mass is covered by a layer of tropho- 

 blastic, often referred to as Rauber's layer. Thus over this area the 

 blastocyst is trilaminar. The blastocyst as a whole continues to 

 grow during this period, and is kept turgid by the infiltration of a 

 fluid produced by a series of glands in the uterine wall. 



The fate of Rauber's layer is not the same in different groups, and 

 the, transformations in its neighbourhood may be complex. In the 

 bats, many rodents and perhaps man, this layer persists and becomes 

 separated 'from the embryonal shield by a cavity designated the 

 primitive ammotic cavity, since it is destined later to give rise to the 

 true amniotic cavity. In other species, notably the rabbit, it appears 

 as if the rapid growth of the embryonal shield, which fuses with the 

 trophoblast around its edges, stretches the layer until it breaks 

 down into a number of loose cells lying under the zona and between 

 it and the shield. These cells may, perhaps, merge in the ectoderm 

 of the shield, but at any rate they disappear as separate structures, 

 and so the blastocyst becomes bilaminar throughout. It will be 

 clear, however, that while the greater part of its periphery is 



