EXTRA-EMBRYONIC MEMBRANES 487 



THE AMNION AND THE SEROSA 



While the splanchnopleure forms the yolk-sac^ it is the somato- 

 pleure, lying outside the embryo, from which both amnion and serosa are 

 derived. 



At about thirty hours, the first observable portions of the amnion 

 appear as a crescentic fold with the concavity toward the head of the 

 embryo. This fold must not be confused with the head fold of the chick 

 which folds under the embryo. 



The head at this time sinks into the yolk to a slight degree, just as 

 the extra-embryonic somatopleure anterior to the head is thrown into 

 the head-fold of the amnion. As the embryo grows anteriorly and the 

 somatopleure caudally, the amniotic fold which is thus folded upon itself, 

 forms a double-walled cap over the head of the embryo, gradually ex- 

 tending more and more caudad. The caudally directed limbs of the 

 head-fold of the amnion continue growing posteriorly on each side of 

 the embryo, where they are known as lateral amniotic folds. These grow 

 dorsad and mesiad, finally meeting in the midline. 



During the third day, the amniotic tail-fold develops and grows 

 cephalad to meet the structures just mentioned, thus forming a complete 

 envelope for the embryo. The place where the various amniotic folds 

 meet is called the amniotic raphe. 



The amnion is now a completed saccular structure filled with a 

 fluid in which the embryo is free to move about and change positions. 

 In all probability, this ability of free movement also prevents adhesions 

 and consequent malformations. 



It is to be noted that the manner in which the amniotic folds came 

 into existence has caused the innermost portions to be ectodermal. This 

 ectodermal layer is continuous with the ectoderm of the embryo. 



Likewise, the manner of the somatopleure folding upon itself, as it 

 does, causes two walls to cover the embryo. The inner one is the ecto- 

 dermal layer just mentioned and the outer one is known as the serosa. 

 There is a sero-amniotic cavity between the two. 



The somatopleure now extends peripherally until the entire yolk- 

 sac, and eventually the embryo as well, is covered with serosa. 



The allantois extends between serosa and amnion. 



THE ALLANTOIS 



This structure differs from the amnion and yolk-sac in that it de- 

 velops within the embryo proper, though it does extend out into the 

 extra-embryonic region as it develops. 



About the third day, the allantois develops by an outpushing of the 

 ventral wall of the hind-gut (entoderm), pushing the splanchnopleure 

 ahead of it, so that we may say it is composed of splanchnopleure with 

 an entodermal lining. The following day it pushes out of the embryo 



