436 OUTLINES OF CHORDATE DEVELOPMENT 



endodermal lining of the yolk-sac, from which at this time the 

 hind-gut is not distinguishable (Fig. 161, D, E). 



The essential relation thus established is never extensively 

 altered. When the hind-gut forms, it is dorsal or postero- 

 dorsal to the opening of the allantoic canal, and as the body 

 stalk elongates the allantoic canal continues to extend up 

 through it, finally reaching the region of the chorionic meso- 

 derm (Fig. 179). The allantois never expands into a free vesicle 

 in the exoccelom, but remains as a vestigial structure, wholly 

 embedded within the tissues of the body stalk, or of what is 

 later the umbilical stalk (see above). The endodermally lined 

 allantoic canal within the umbilical cord, remains present in 

 this condition throughout the foetal period. As the ventral 

 body wall of the embryo is formed it encloses the proximal 

 portion of the allantoic stalk, and this becomes enlarged, 

 forming the rudiment of the urino-genital sinus and the urinary 

 bladder; between this and the body wall it is reduced to a solid 

 strand of connective tissue called the urachus. 



The blood vessels of the allantois (umbilical arteries and 

 veins) remain typically developed here, in spite of the 

 vestigial character of the endodermal portions of the allantois, 

 and as in the rabbit these are the only functional elements of 

 the whole allantoic structure. The vessels are very large and 

 form a very rich network in the placental region, beyond the 

 limits of which the chorion becomes almost non-vascular, 

 although in earlier stages the entire chorion is vascular. 



Only among the higher Primates is the allantois vestigial to 

 such a degree; and it is not often that it has as limited an extent 

 as in the rabbit. In other forms, such as the Lemurs, Carnivors, 

 and Ungulates, the allantois extends completely around the 

 inner surface of the chorionic vesicle; among the Ungulates 

 this seems to be correlated with the simple type of placenta 

 (see below). In such cases, and in some other instances 

 where the allantois is more nearly limited to a definite placental 

 region, a definite allantoic cavity is present, small and com- 

 pressed in forms like the sheep and pig, or large and filling a 

 large part of the cavity of the entire vesicle. 



