296 Vertebrate Embryology 



its exact mode of origin in mammals is, perhaps, 

 uncertain, but it probably originates by delami- 

 nation from the upper side of the entoderm. 

 In some forms the mesoderm extends entirely 

 around the blastodermic vesicle, in others only 

 a part of the way. 



Extending forward from the primitive knot 

 there now appears a linear thickening that, 

 superficially, has somewhat the same appear- 

 ance as the primitive streak ; this is the primi- 

 tive axis, the long axis of the future embryo. 

 The primitive axis gradually extends backward 

 and encroaches upon the primitive streak, until 

 the latter structure is obliterated. 



At this stage of its development the blasto- 

 dermic vesicle, with its embryonic shield, cor- 

 responds to the ovum of a chick of about 18 to 

 20 hours, except that in the chick the large yolk 

 sac is filled with yolk, while in the mammal 

 it is filled with a liquid. 



The origin of the notochord, neural canal, 

 and other embryonic structures is about the 

 same as has been studied in the chick, so that 

 if these processes be understood in the latter 

 type of development, they will be readily com- 

 prehended in the development of the mammal. 

 The folding off of the embryo from the rest 



