DEVELOPMENT OF THE FCETAL MEMBRANES. 



43 



attached to the uterine mucous membrane, and these villi subsequently become 

 ramified and vascular when the growth of the allantois has brought the umbilical 

 blood-vessels to the chorion ; but except in the placenta, they at length all become 

 atrophied and disappear. 



It will be seen from the manner in which the true and false amnion are formed 

 by a fold of somatopleure, that these membranes are composed of both epiblast and 

 inesoblast. In the false amnion the epiblast becomes the outer layer ; in the true 

 amnion it is the inner layer. The inesoblast of the one is separated from that of 

 the other by a space occupied by fluid, 1 and continuous with the ccelom, with which 



Fig. 45. DIAGRAM OF A LONGITUDINAL SECTION OF A MAMMALIAN EMBRYO, AFTER THE COMPLETION 



OF THE AMNION. 



in fact it remains continuous until the body walls of the embryo have entirely grown 

 round and coalesced on the ventral surface the final point of coalescence being the 

 umbilicus. With this enclosing growth of the body walls the line of reflection of 

 the amniotic fold is a] so carried downwards, so that the amnion is eventually 

 attached around the umbilical cord, by which the foetus appears suspended in the 

 amniotic fluid (fig. 50). 



In the guinea-pig 1 , in which the epiblast is the innermost layer, the amnion is not formed 

 as a fold, but results from an extension of the mesoblastic cleavage around the dorsal aspect 

 of the central cavity ; this cavity thus becomes the cavity of the amnion. 



Formation of the allantois. Both the amnion and the chorion are entirely 

 extra-embryonic structures, i.e., they are external to the body of the embryo, and, 



will be used throughout this article in the sense in which it has hitherto almost invariably been 

 employed in human embryology, to denote that external membrane of the ovum from which the 

 villi (chorionic villi) which grow into the uterine mucous membrane spring, and this it will be seen 

 presently, is that part, of the external investment of the blastodermic vesicle, which, when the amnion is 

 formed, becomes the external amniotic fold or false amnion. 



1 Except in the later stages of gestation, when the amniotic and chorionic inesoblast become loosely 

 united by jelly-like connective tissue. 



