328 EMBRYOLOGY. 



edges are in mutual contact. The closure of the sac takes place in 

 a somewhat different manner from that of the Chick. Instead of 

 meeting in a longitudinal suture, the edges of the amniotic folds 

 meet, in the Rabbit at least, approximately in the middle of the back 

 in a small spot, where for a considerable time a circular opening in 

 the sac is retained. The outer layer of the amniotic fold, which in 

 diagram 3 is still in connection with the amniotic sac at the point of 

 fusion, but which later entirely separates from it, represents, as in 

 the Chick, the serosa. It first appears as an independent structure 

 in the vicinity of the embryo, whereas farther downwards it is still 

 firmly united with the entoblast, and together with it constitutes the 

 wall of the original blastula, which is here only two-layered. 



In the third diagram, furthermore, we can recognise the first 

 trace of the allantois (al), which grows out from the anterior 

 wall of the hind gut in the manner already described (p. 217), and 

 which in the Rabbit is seen as early as the ninth day in the form of 

 a small, pedunculated, exceedingly vascular sac. 



The fourth diagram shows the development of the f oetal membranes 

 much further advanced. The prochorion has become ruptured by 

 the distension of the entire blastodermic vesicle, and is no longer 

 recognisable as a separate membrane. What we see on the outside 

 is the serosa, which has been changed in a striking manner. In the 

 first place, it has become completely detached from the amnion ; 

 however, it should be remarked in this connection that in certain 

 Mammals, and especially in Man, a stalk uniting the two membranes 

 is retained for a considerable time at the amniotic suture. Secondly, 

 the serosa is everywhere separated from the yolk-sac, and loosely 

 surrounds the embryo and its remaining membranes as a thin sac. 

 This condition has been brought about in the following manner : the 

 middle germ-layer, which in diagram 3 had grown over only one half 

 of the original blastula, has now spread over the other half also, and 

 has become divided into its two layers. By this means the extra- 

 embryonic part of the blastula is now completely split, as in the Chick, 

 into an outer sac, the serosa, and the yolk-sac, separated from it 

 only by the body-cavity. 



Moreover, there exist in this respect differences among the 

 Mammalia, since in some the serosa remains to a greater or less 

 extent permanently united with the yolk-sac. This is the case, for 

 example, in the Rabbit. 



In the Rabbit, in which the yolk-sac at first fills the greater part of the 

 blastodennic vesicle, the middle germ-layer spreads out over that half of the 



