230 THE DEVELOPMENT OF THE CHICK 



epithelium over the entire extent of the septa and forms loops 

 along the free border. The arteries do not communicate directly 

 with this network according to Popoff , and the course of the 

 circulation from arteries to veins is not clearly described by this 

 author. 



The allantois fuses with the yolk-sac in the region of the 

 yolk-sac umbilicus, and anastomoses arise between the veins of 

 the allantois and those of the yolk-sac. 



Ultimate Fate of the Yolk-sac. On the nineteenth day of 

 incubation, the yolk-sac slips into the body-cavity through the 

 umbilicus; which thereupon closes. The mechanism of this 

 process is of considerable interest. The yolk-sac is still a volu- 

 minous organ, and equal to about one sixth the weight of the 

 embryo. It is therefore inconceivable that it could be "drawn 

 into" the body-cavity by means of its stalk, which has only the 

 intestine for attachment. The process is much more complex 

 and may be briefly described as follows: We have already seen 

 that the inner wall of the allantois fuses with the amnion on the 

 one hand; distally it is connected with the yolk-sac. Now this 

 wall of the allantois is muscular, and it is probable that its con- 

 traction is the first act in the inclusion of the yolk-sac within the 

 body-wall. It is aided in this, however, by the inner wall of 

 the amnion, i.e., that part of the amnion arising from the umbili- 

 cus and not fused with the allantois. This part of the amnion 

 surrounds the yolk-stalk, and is itself richly provided with muscle 

 cells, forming a crossing and interlacing system. It is carried 

 down and over the yolk-sac to about its equator by the allantois, 

 and when the yolk-sac is half taken into the body-cavity, it reaches 

 its distal pole and fuses there. Now if the egg be opened at 

 this stage in the process and this wall of the amnion cut through, 

 it contracts rapidly to a fraction of its former area (Virchow). 

 It is apparent, then, that the tension of this membrane on the 

 yolk-sac must exert a continuous pressure that tends to force it 

 into the body-cavity. It is in this way, then, by contraction 

 of the inner walls of the allantois and of the amnion, that the 

 yolk-sac is pressed into the body-cavity. 



The umbilicus is therefore closed b}- the mere act of inclusion 

 of the yolk-sac, for the inner amniotic wall is attached on the 

 one hand to the body-wall, and on the other to the distal pole 

 of the yolk-sac. A minute opening is left in the center of the 



