THE FCETAL MEMBRANES OF REPTILES AND BIRDS. 219 



is acquired in part directly through the egg-shell and in part out of 

 the air chamber (fig. 8 a.ch) situated at the blunt pole of the egg, 

 which is in contact with a large part of the allantois. 



Finally, in addition to respiration, the allantois serves for the 

 resorption of the albumen, which becomes more and more thickened 

 during incubation, and compressed into a lump at the pointed pole of 

 the egg. It grows over the albumen and envelops it in a sac, the epi- 

 thelial surface of which arose from the serosa, which was evaginated 

 at the same time with the growing allantois. There are developed on 

 the inner surface of the sac highly vascular villi, which sink into the 

 albumen, and have been described as a placenta by DUVAL, who has 

 called attention to these conditions. 



The air chamber also has undergone modifications during incuba- 

 tion, and, at the same time with the acquisition of air, has increased 

 in size by the separation of the two layers of the shell-membrane in 

 which it is enclosed (fig. 8, p. 17). 



Finally, the amnion, which at the beginning of its development is 

 rather closely applied to the embryo, has enlarged and become a sac 

 (Plate I., fig. 5 A) entirely filled with amniotic fluid. Its rhythmical 

 contractions already described become most active and powerful on 

 the eighth day, and from that time forward to the end of incubation 

 diminish in frequency and in force. 



As a result of all these processes of growth, the embryo with its 

 appendages now demands a much larger space than at the beginning 

 of incubation. It acquires this in the following manner. The 

 albumen which surrounds the yolk diminishes considerably, since it 

 disappears, especially its fluid portion, partly by evaporation to the 

 exterior, partly also by resorption on the part of the embryo. The 

 vitelline membrane has become ruptured by the enlargement. 



In the second period, which we have reckoned from the eleventh 

 to the twenty-first day, or to the hatching of the Chick, retrogressive 

 metamorphoses are most prominent. 



These assert themselves first of all on the yolk-sac. As the result 

 of the vigorous sucking up of its contents it becomes more and more 

 flaccid, so that its wall begins to lie in folds. It now becomes 

 entirely separated from the serosa, since the extra-embryonic body- 

 cavity has extended all around it, and thereupon it is drawn closer to 

 the wall of the belly by the shortening of the umbilical stalk. On 

 the nineteenth day of incubation it begins to slip into the peritoneal 

 cavity through the dermal umbilicus, which has now become very 

 narrow, whereby it takes on an hour-glass shape during its passage 



