THE EARLY DEVELOPMENT OF THE CHICK 299 



branes forming its outer wall, the sero-amnionic fusion becomes 

 perforated, and the albumenous material partly passes into 

 solution in the amnionic fluid. For the most part, however, 

 the albumen is absorbed through the walls of the albumen sac. 

 This is fully accomplished toward the close of the embryonic 

 period, and the albumen sac then remains as an appendage of 

 the yolk-sac, with which it passes into the body of the embryo 

 at the end of incubation. 



The most characteristic feature of the allantois is the extreme 

 vascularity of its outer wall. The withdrawal of the albumen 

 from the greater part of its surface leaves the fused chorion 

 and outer allantoic wall in direct apposition to the shell mem- 

 brane. The chorionic layers become extremely thin and 

 the shell membrane becomes very porous, so that there is an 

 easy exchange of gases between the outside air and the capil- 

 laries of the outer allantoic wall. These capillaries are very 

 wide and extremely abundant, so altogether the allantois is a 

 very efficient respiratory organ. 



The inner wall of the allantois, as already mentioned, fuses 

 with the amnion in the region dorsal to the embryo, during the 

 second week of incubation. Here as elsewhere in the inner 

 wall, muscle fibers may develop. Later on, as the period of 

 incubation draws toward its close, it fuses also with the yolk- 

 sac. The cavity of the allantois remains connected with the 

 cloaca by the tubular allantoic stalk. It is filled with a fluid 

 containing the excretory products, received from the cloaca 

 where the excretory ducts discharge. 



Von Baer's account of the principal events associated with 

 the hatching of the young chick, is summarized in the two 

 following paragraphs quoted from Lillie (" Development of the 

 Chick," p. 232). 



" About the fourteenth day the growing embryo accommo- 

 dates itself to the form of the egg so as to lie parallel to the long 

 axis with its head usually toward the broad end near to the air- 

 chamber. Sometimes, however, the embryo is turned in the 

 reverse position (Von Baer). The head is bent toward the 

 breast, and is usually tucked under the right wing. Important 



