230 EMBRYOLOGY. 



that the middle germ-layer is wanting in the region of the anterior amniotic fold 

 to a greater degree in this case than in the Chick. The anterior amniotic fold 

 therefore consists during a considerable period of only the two primitive germ- 

 layers, closely joined together. VAN BENEDEN has therefore given to the 

 cephalic sheath, as long as the inner germ-layer takes part in its formation, 

 the name of proamnion. Later on, however, a separation of the amnion i'mm 

 the entoblast takes place also in the head-region in the Rabbit. 



Finally, in our fourth diagram, still a third change has appeared in 

 the serosa. By rapid growth of the epithelium large numbers of 

 small evaginations or villi have arisen on its outer surface. On this 

 account the name of chorion or mllous layer has been applied to it 

 when these changes have been completed. It should also be added 

 here that in the development of the villi uniformity among all Mammals 

 by no means prevails. In the lowest orders (Monotremes, Marsupials) 

 the surface of the blastodermic vesicle remains almost smooth, as in 

 Reptiles and Birds. Ill them, therefore, the serosa is permanently 

 retained during embr} T onic life, whereas in other Mammalia it is 

 transformed into a villous membrane. By reason of these differences 

 KOLLIKER has divided Mammals into Mammalia achoria and 

 Mammalia choriata. 



On the other embryonic membranes of fig. 132, 4, it is principally 

 changes in size only that have been effected. The yolk-sac (ds), over 

 the entire surface of which the vitelline vessels now spread, has 

 become considerably smaller, and is continuous with the embryonic 

 intestine by means of a long slender stalk, the vitelline duct (da). 

 The amniotic sac (am) has already enlarged and is filled with fluid, 

 the liquor amnii. Its walls are continuous at the umbilicus with 

 the ventral wall of the embryo. The allantois (at) has become a 

 vascular pear-shaped sac, which has grown out between the dermal 

 stalk and umbilicus into the extra-embryonic part of the body-cavity, 

 and soon after reaches the serosa. 



The accurate representation of an embryo Dog of twenty-five days 

 (fig. 134) affords us, better than the diagram (fig. 132, 4), a view of 

 the connection of the two vascular sacs, the allantois and yolk-sac, 

 with the intestinal canal. 



The embryo is removed from the chorion and amnion. The 

 ventral belly- wall is partly removed, and thereby the dermal um- 

 bilicus, which about this time has become rather narrow, has been 

 destroyed. The intestinal canal, now to be seen in its entire length. 

 is already converted throughout into a tube (d) ; near its middle it is 

 continuous, by means of a short vitelline duct, with the yolk-sac 



