160 



ALLAN TOTS. 



As the hindgut becomes folded in the allantois shifts its position, 

 and forms (figs. 123 B and 124) a rather wide vesicle lying imme- 

 diately below the hind end of the digestive canal, with which it 

 communicates freely by a still considerable opening ; its blind end 

 projects into the pleuroperitoneal cavity below. 



Still later the allantois grows forward, and becomes a large 

 spherical vesicle, still however remaining connected with the cloaca 

 by a narrow canal which forms its neck or stalk (fig. 121 G, al). 

 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 en- 

 close the yolk-sack. 

 It does not during 

 the third day pro- 

 ject beyond this 

 point ; but on the 

 fourth day begins 

 to pass out beyond 

 the body of the 

 chick, along the as 

 yet wide space be- 

 tween the splanch- 

 nic and somatic 

 stalks of the em- 

 bryo, on its way to 

 the space between the external and internal folds of the amnion, 

 which it will be remembered is directly continuous with the pleuro- 

 peritoneal cavity (fig. 121 K). In this space it eventually spreads 

 out over the whole body of the chick. On the first half of the fourth 

 day the vesicle is still very small, and its growth is not very rapid. Its 

 mesoblast wall still remains very thick. In the latter half of the day 

 its growth becomes very rapid, and it forms a very conspicuous object 

 in a chick of that date (fig. 118, Al). At the same time its blood- 

 vessels become important. It receives its supply of blood from two 

 branches of the iliac arteries known as the allantoic arteries \ and the 

 blood is brought back from it by two allantoic veins which run 

 along in the body walls (fig, 119) and after uniting into a single trunk 

 fall into the vitelline vein close behind the liver. 



Before dealing with the later history of the foetal membranes, it 

 will be convenient to complete the history of the yolk sack. 



Fig. 124. Diagbammatic longitudinal section through 



THE POSTEKIOR END OF AN EmBRTO BiRD AT THE TIME OF 

 THE FORMATION OF THE AlLANTOIS. 



ep. epiblast; Sp.c. spinal canal; ch. notochord; n.e. 

 neurenteric canal; hy. hypoblast; p.a.g. postanal gut; pr. 

 remains of primitive streak folded in on the ventral side; 

 al. allantois ; me. mesoblast ; an. point where anus will be 

 formed; p.c. perivisceral cavity ; am, amnion; so. somato- 

 pleure; sp. splanchnopleure. 



1 T propose to call these arteries and the corresponding veins the allantoic arteries 

 and veins, instead of using the confusing term 'umbilical.' 



