THE EARLY DEVELOPMENT OF THE MAMMAL 417 



IV. THE EMBRYONIC MEMBRANES AND APPEND- 

 AGES OF THE EUTHERIAN MAMMALS 



Scattered references have been made in the preceding pages 

 to various details regarding the development of the amnion, 

 the chorion, the allantois, and yolk-sac, and we must now give, 

 possibly with some repetition, a more connected, though brief, 

 account of the development of these structures. There is in 

 general a remarkable similarity between the early history of 

 the embryonic appendages of the Mammals and those of the 

 Sauropsida, a similarity that is the more remarkable when the 

 eggs of the two groups are compared. As mentioned in the 

 introductory paragraphs of this chapter, these similarities are 

 to be explained upon an historical basis, that of relationship 

 through descent. But while there is, in the early stages, such 

 close agreement between Mammal and Sauropsid, in these re- 

 spects, during their later history, these mammalian membranes 

 undergo profound changes in function associated with the intra- 

 uterine development of the embryo and the consequent sub- 

 stitution, as a source of nutritive substance, of the maternal 

 tissues in place of the intra-oval yolk-mass or albuminous egg 

 membranes of the Sauropsida. 



In the chick the amnion serves, among other functions, to 

 protect the embryo from drying and from the deforming pres- 

 sure of the rigid shell; the yolk-sac contains a large part of 

 the food substance for the developing embryo; the allantoic 



