150 THE FOUKTH DAY. [CHAP. 



its position and becomes quite parallel with the commencing digestive tract. 

 Its cavity is separated from that of the digestive canal by a projection of 

 mesoblast covered by hypoblast ; but both open freely in front into the common 

 splanchnic stalk. 



In the next stage it still further alters its position, and forms, Fig. 50, a rather 

 wide vesicle lying immediately below the hind end of the digestive canal, with 

 which it communicates freely by a still broad opening; its blind end projects 

 freely into the pleuroperit.oneal cavity below. It was in this condition when 

 Von Baer first observed it. 



At the time when these changes are taking place, the somatopleure is 

 being folded in to form the walls of the body; and as the folds, one on either 

 side, are carried forward from the extreme end of the tail, they present them- 

 selves, when seen from within or in sections, as two ridges projecting towards 

 the sides of the allantois. Reaching the allantois these ridges fuse with its wall, 

 and in this way reduce the pleuroperitoneal cavity immediately below the allan- 

 tois to quite a narrow space, which is seen in section as a mere chink. Remak 

 apparently mistook these infoldings of the somatopleure, and the consequent 

 projections into the pleuroperitoneal cavity, for the first formation of the 

 allantois, although they have in fact little or no connection with it. 



We may therefore probably consider the allantois as a 

 portion of the cloaca, which grows forward and becomes an 

 independent spherical vesicle, still however remaining con- 

 nected with the cloaca by a narrow canal which forms its neck 

 or stalk. The opening of the allantois into the cloaca is on 

 the under side of the latter. Both the neck and vesicle of 

 the allantois are lined by hypoblast, while its exterior is com- 

 posed of the mesoblast of the splanchnopleure. From the 

 first the allantois lies in the pleuroperitoneal cavity. In this 

 cavity it grows forwards till it reaches the front limit of the 

 hind-gut, where the splanchnopleure turns back to reach 

 the yolk-sac. It does not during the third day project be- 

 yond this point ; but on the fourth day begins to pass out 

 beyond the body of the chick along the as yet wide space 

 between the splanchnic and somatic stalks of the embryo 

 on its way to that space between the external and internal 

 folds of the amnion, which it will be remembered is directly 

 continuous with the pleuroperitoneal cavity (Fig 8, K). In 

 this space it eventually spreads out over the whole body of 

 the chick. On the first half of the day the vesicle is still 

 very small, and its growth is not very rapid. Its mesoblast 

 wall still remains very thick. In the later half of the day its 

 growth becomes very rapid and it forms a very conspicuous 

 object in a chick of that date (Fig. 46, AL). At the same 

 time its blood-vessels become important. To these we shall 

 presently return. 



