92 EARLY EMBRYOLOGY OF THE CHICK 



Lwith the serosa is of particular interest because of its homology , 

 ^wwith the establishment of the chorion in the higher mammals.^ ^ 

 JThe chorion of mammalian embryos arises by the fusion of 

 allantoic vessels and mesoderm with the inner wall of the 

 serosa, and constitutes the embryos's organ of attachment to 

 the uterine wall. In mammaHan embryos the allantoic, or 

 umbilical circulation as it is usually called in mammals, serves 

 more than a respiratory function. In the absence of any 

 appreciable amount of yolk, the mammalian embryo derives its , 

 (nutrition through the allantoic circulation from the uterine / 

 blood of the mother. Thus the mammalian allantoic circula- 

 tion carries out the functions which in the chick are divided 

 between the vitelline and the allantoic circulations. 



^ By reason of this homology the serosa of the chick is sometimes called chorion. 

 It seems less likely to lead to confusion if the use of the term chorion is re- 

 stricted to mammalian forms, especially as the serosa alone is the homologue 

 of only part of the mammalian chorion. In some books the term outer or false 

 amnion will be found used to designate the structure called serosa in this book. 

 The term false amnion is not, however, in general use in this country.' 



