SOCIETY OF THE UNIVERSITY OF ABERDEEN. 105 



caudally between the aortic arches (Fig. 8). The lung anlage is not 

 plainly indicated, but caudal to the median thyroid is a groove in the 

 entoderm which may indicate the lung groove. As the fore gut is 

 traced towards the yolk sac, its lumen becomes narrowed transversely 

 and elongated dorso-ventrally ; and just before it opens out into the 

 yolk sac, there is a liver bay with walls of thickened entoderm. There 

 is no actual liver bud and no trace of liver trabeculaj. The middle 

 third of the alimentary tract is still in communication with the yolk 

 sac. The hind gut is narrowed at a point opposite the attachment of 

 the body stalk, and then dilates to form a large cloaca. Dorsally the 



a AO. 



Fig. 8. Section through region of hind-brain, x 50. 



/'/'., fore-brain; Ab., hind-brain; ffy., hypophysis; Ch., 

 notochord ; I'g. , vagus ganglion ; d.Ao., dorsal aorta ; 

 Ph., pharynx ; T.A., aortic stem ; Th., median 

 thyroid. 



cloaca is expanded with convex walls, while ventrally it is narrowed 

 so that its cavity forms a fissure. Ventrally the entoderm of this 

 anterior fissure-like part of cloaca is in direct contact with the 

 ectoderm, forming an elongated cloacal membrane. Cranial to the 

 cloacal membrane the cavity of the cloaca leads into the small lumen 

 of the allantois. The allantoic duct passes into the body stalk between 

 the two umbilical arteries, which eventually surround it by anastomos- 

 ing across it. There is a very small post-anal gut projecting caudally 

 between the posterior limit of the cloacal membrane and the tip of the 



notochord. 



II 



