538 VERTEBRATE LIFE AND ORGANIZATION 



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B 



(Peripheral cells of D and 

 E are arranging in a layer, 

 while fluid vacuoles are 

 appearing between the in- 

 ternal cells. F and G are 

 parts of blastocysts whose 

 proportions are shown in 

 small, outline drawings.) 



D 



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Figure 31.1. Photomicrographs of cleavage in mammalian eggs developing in a 

 tissue culture. A-C, Two, four and eight-celled stages of the monkey; D, morula of a 

 rabbit; E-G, blastocysts of a rabbit. Observe the thick membrane that surrounds the 

 early stages. Several sperm are entrapped in this in A and B. (After Lewis, Hartman 

 and Gregory.) 



This stage, known as the blastocyst, can be compared to the blastula 

 of other vertebrates. However, only a group of cells at one pole of the 

 blastocyst, the Inner cell mass, forms the embryo (Fig. 31.2). The 

 peripheral layer of cells, known as the trophoblast, comes in contact 

 with the uterine lining and begins to form a placenta before the 

 embryo itself has developed to any great extent. The value of the 

 precocious development of this layer in a yolkless embryo that is not 

 free to forage for itself is obvious. The trophoblast is comparable to 

 the ectoderm of the chorion, which is the outermost of the extra- 

 embryonic membranes of all amniotes. Bushy projections called villi 

 develop on its surface and penetrate the uterine lining in most mam- 



