VL] 



THE ALLANTOIS. 149 



the wall of the cloaca: with this they unite, and form together a solid 

 spherical body, bearing on its external surface a median furrow, indicating its 

 double origin. A narrow diverticulum of hypoblast now passes into the mass, 

 and forms within it a cavity, which is at first small and, corresponding to the ex- 

 ternal contour of the body, to a certain extent double. The hypoblast diverticulum 

 grows rapidly, while its mesoblastic covering remains nearly stationary, so that 

 the mesoblast finally comes to form a thin coating only over the hypoblast. 



His (op. cit. ) gives a somewhat elaborate and complicated account of the 

 development of the allantois; which is accepted by Waldeyer (Eierstock und 

 Ei) and Bornhaupt (Untersuckuny iiber die Entwickelung des Urino-genitalsy stems 

 beim Hiihnchen, Riga, 1867). 



It appears to be nearly the same as the fuller account given by Dobrynin 

 (Ueber die erste Anlage der Allantois. Sitz. der k. Akad. Wien, Bd. 64, 1871), 

 of which the following is an abstract. 



When the first commencement of the hind fold takes place, immediately 

 beyond the point where the hypoblast turns back to assume its normal direction 

 over the yolk-sac, a narrow diverticulum which points backwards and some- 

 what upwards is formed by a special flexure of the splanchnopleure. The open 

 end of the diverticulum, Fig. 49, All. f looks forwards towards the wide opening 

 connecting the digestive tract with the yolk-sac; its blind end points directly 

 towards the pleuroperitoneal cavity. This diverticulum is the commencing 

 allantois. It is lined by hypoblast, while its exterior is composed of the mesoblast 

 of the splanchnopleure. 



As the folding in to form the digestive tract increases, the diverticulum alters 



FIG. 50* 



LONGITUDINAL SECTION OP THE TAIL-END OF AN EMBRYO CHICK AT THE 

 MIDDLE OF THE THIRD DAY (Dobrynin). 



t. the tail; the line of reference points to the axial mesoblast at the tail. 

 x. epiblast. SO. somatopleure. ra. mesoblast to form the body wall. 

 V. commencing amniotic fold. Hp. space between the true and the false 

 amnion. pp. Pleuroperitoneal cavity. Spl. splanchnopleure. G. Cloacal 

 enlargement of the alimentary canal. Dd. dorsal wall of the alimentary 

 canal. All. vesicle of the allantois having a wide opening into the alimen- 

 tary canal. 



