338 PROFESSORS ARTHUR ROBINSON AND A. GIBSON. 



The dorsal ends of all four arches fade away, on the dorsal aspect of the hind-brain 

 region of the head. 



On the inner surface of the ventral end of each first arch there is a slight ledge- 

 like projection which runs along the corresponding cranio-lateral border of the 

 tuberculum impar and fuses in front of the tuberculum with its fellow of the opposite 

 side. These ledges probably represent the rudiments of the lateral borders and the 

 tip of the tongue (fig. 63, PI. XIX). 



The inner portions of the ventral ends of the second arches are fused with one 

 another across the floor of the pharynx, and the ventral ends of the inner portions 

 of the third arches end in slight elevations, on the floor of the pharynx, but they 

 are not yet united together in the median plane (fig. 63, PL XIX). 



Each first and second branchial arch is traversed by the corresponding aortic arch, 

 but no definite blood-vessels are present in the third and fourth arches. Their cores 

 are formed by thickened bars of undifferentiated mesoderm interposed between the 

 ectodermal and entodermal layers. 



Mid-gut and Hind-gut. 



There is no definite mid-gut, and only a small part of defined hind-gut. For a 

 distance corresponding with two and a half millimetres of the embryo, caudal to the 

 cranial border of the umbilicus — that is, caudal to the posterior border of the peri- 

 cardial region — there is no trace of demarcation of gut from yolk-sac, but more 

 caudally there is a cleft 400/x long, in the embryo, which leads into a definitely 

 enclosed hind-gut (fig. 55, PI. XIX). 



It should be noted that the cleft is caused by infolding of the lateral parts of 

 the splanchnopleure, and that the small cloacal chamber and the allantoic diverti- 

 culum have either been enclosed by the meeting and fusion of the lateral folds or 

 they are diverticula from the yolk-sac, for there is no trace of any caudal fold. 



There can be scarcely any doubt that, of the two diverticula from the caudal end 

 of the hind-gut, the one which runs caudally is the allantoic diverticulum, and that 

 which passes dorsally is either a part or the whole of the rudiment of the cloacal 

 chamber ; but it is noteworthy that, of the two, the allantoic diverticulum at this 

 stage approaches nearer to the amnion cavity than does the cloacal diverticulum, 

 and that the cloacal membrane is a comparatively thick mass of tissue, with but 

 small transverse or cranio-caudal extension (text-fig. 23, p. 335, and fig. 57, PI. XIX). 



The Entodermal Cloaca. 



The pouch-like diverticulum from the dorsal wall of the caudal portion of the 

 gut which represents part or the whole of the rudiment of the cloacal chamber has a 

 cavity which is only SOjjl long in the embryo. Its entodermal epithelial wall is thick, 

 and at its caudal end it fuses, dorsally, with a terminal mass of cells in which end the 



