EMBRYO FORMATION IN OTHER GROUPS OF VERTEBRATES 237 



known of this in detail and no vital marking experiments have yet been 

 possible; there may well be some migration from the edges of the inner 

 cell mass along the base of it towards the centre. 



The mesoderm is formed from the outer surface of the iimer cell masss. 

 In this region the outermost layer of all, the 'enveloping layer' which is 

 continuous with the main sphere of the blastocyst, either lifts away to 

 leave a cavity (thus forming an amnion), or breaks dov^i and disappears. 

 The face of the inner cell mass thus exposed has in general an oval form 

 in plan view, and on it there appears an elongated thickening, wliich is 

 the primitive streak. In some forms, such as the rabbit, it is extremely like 



Figure 11.7 



Early development in a mammal. A, section of blastocyst, with itmer cell 

 mass. B, later stage, the main part of the blastocyst becoming the tropho- 

 blast (Tr), the lower layer of the inner cell mass becoming arranged as an 

 epithelium, the endoderm. C, the iimer cell mass arranged as two epithelia, 

 ectoderm and endoderm; above the latter the original outer layer has be- 

 come elevated so as to form the amniotic cavity {R.in., 'Rauber's mem- 

 brane'). 



that seen in birds; in others, such as the mole and in man, the streak is 

 shorter and there is a greater development of a chorda-mesodermal canal 

 such as that found in reptiles. The invagination of the mesoderm has never 

 been followed in detail by vital staining, but a few cinematograph fdms 

 of rabbit blastoderms growing in vitro have been taken by Waddington, 

 and a convergence of lateral material towards the streak from both sides 

 has been seen ; there is little doubt that the process is essentially similar to 

 that of the bird and reptile. There are, of course, certain differences in 

 detail. Thus in the rabbit the presumptive neural plate probably occupies 

 a greater proportion of the embryonic area which corresponds to the area 

 pellucida of the chick; and Waddington (1937) points out that the results 

 of cultivation in vitro of posterior halves of rabbit embryonic areas 



