XIV ELEMENTS OF VERTEBRATE EMBRYOLOGY 475 



cavity from the remainder of the space in the interior of the inner 

 mass. 



In the Mouse there is still greater delay, for the ectodermal part of the 

 inner mass remains solid until a period corresponding to that in the Vole 

 when the amniotic folds have already met. It is only then that two 

 cavities develop, one over the other, along the centre of the inner mass. 

 These two cavities become for a time continuous (Fig. 199, B), but 

 eventually the lower part becomes separated off to form the definitive 

 amniotic cavity while the upper portion becomes obliterated. 



It seems clear from the few known early stages of Human develop- 

 ment, such as the Bryce-Teacher blastocyst, that in Man also the amniotic 

 cavity arises as a closed space within the inner mass. A striking feature 

 in the early blastocyst of Man and of Apes (Fig. 199, C) is the great 

 reduction in the size of the yolk-sac (y.s), correlated with a precociously 

 developed wide coelomic space (c) separating it from the wall of the 

 blastocyst. 



It will have been gathered that in the Amniota in general the young 

 individual during its embryonic development is enclosed in a water jacket, 

 the amnion, and that this in turn is enclosed within an outer wall of 

 somatopleure constituted by the false amnion and its continuation, with 

 the inner' surface of which may be fused to varying extents the allantois 

 and yolk-sac. All these structures are referred to collectively as the 

 embryonic membranes or foetal membranes of the amniote. Within these 

 the young individual undergoes gradual development and growth until at 

 last they are ruptured and it is set free. This process of hatching takes 

 place in birds and in most reptiles long after the egg has passed from the 

 body of the mother : in Echidna it takes place within the pouch ; in the 

 typical mammal it takes place within the uterus. 



A long time before the young animal breaks its way out from the 

 foetal membranes muscular movement commences, and in the majority 

 of amniotes, in which sharp claws are present, the movements of the 

 limbs would be liable to tear the delicate amnion and thus bring about the 

 death of the embryo. As was discovered by Agar this danger is guarded 

 against by a beautiful adaptive arrangement, the concavity of the claw 

 being filled up during the period of embryonic existence by a smooth 

 rounded cushion which renders it quite incapable of injuring the amnion. 



