AVES. 191 



sack, or amnion proper (fig. 121, H, I, &c., a\ and the fluid which 

 it afterwards contains is called the amniotic fluid, or liquor 

 amnii. The space between the inner and outer sack is, from 

 the mode of its formation, simply a part of the general cavity 

 found everywhere between somatopleure and splanchnopleure. 

 The outer sack over the embryo lies close under the vitelline 

 membrane, and the cavity between it and the true amnion is 

 gradually extended over the whole yolk-sack. 



The actual manner in which the amniotic folds meet is somewhat 

 peculiar (His and Kolliker). The head-fold of the amnion is the earliest 

 formed, and completely covers over the head before the end of the second 

 day. The side and tail folds are later in developing. The side-folds finally 

 meet in the dorsal line, and their coalescence proceeds backwards from the 

 head-fold in a linear direction, till there is only a small opening left over the 

 tail. This also becomes closed early on the third day. 



The allantois ' is essentially a diverticulum of the alimentary 

 tract into which it opens immediately in front of the anus. Its 

 walls are formed of splanchnic mesoblast with blood-vessels, 

 within which is a lining of hypoblast. It becomes a conspicuous 

 object on the third day of incubation, but its first development 

 takes place at an earlier period, and is intimately connected with 

 the formation of the posterior section of the gut. 



At the time of the folding in of the hinder end of the. mesen- 

 teron the splitting of the mesoblast into somatopleure and 

 splanchnopleure has extended up to the border of the hinder 

 division of the primitive streak. As has been already mentioned, 

 the ventral wall of the postanal section of the alimentary tract 

 is formed by the primitive streak. Immediately in front of this 

 is the involution which forms the proctodaeum ; while the wall 

 of the hindgut in front of the anus owes its origin to a folding in 

 of the splanchnopleure. 



The allantois first appears as a protuberance of the splanchno- 

 pleure just in front of the anus. This protuberance arises, how- 

 ever, before the splanchnopleure has begun to be tucked in so as 



1 For details on the development of the allantois the reader is referred to the works 

 of Kolliker (No. 135), Gasser (No. 127), and for a peculiar view on the subject Kupffer 

 (No. 136). In addition to these works he may refer to Dobrynin " Ueber die erste 

 Anlage der Allantois." Sitz. der k. Akad. Wien, Bd. 64, 1871. E. Gasser, Beitriige 

 zur Entwicklungsgeschichte d. Allantois, etc. 



