THE DEVELOPMENT OF VERTEBRATA. 269 



being difficult to follow because, from the first, the hypoblast 

 cells are large, and the cavities which they line, very minute. 

 The gall-bladder arises rather late as an outgrowth of one 

 of the main branches. The pancreas arises as a dorsal out- 

 growth, close behind the point of origin of the liver, and 

 branches in a manner that leads to the diffuse condition in 

 the rabbit, but to a compact gland in frog and chick. In 

 the frog, at a later stage, unequal growth of the parts 

 causes the pancreatic opening to shift its position and open 

 into the bile-duct, as it does in the adult frog. In the 

 rabbit, growth of the duodenum between the two points 

 causes the considerable separation between the two apertures 

 seen in the adult. 



4. Cloaca. In all three types the hind part of the 

 mesenteron, together with the proctodaeum (which is far 

 less extensive than the stomodseum) forms a cloaca. This 

 persists in frog and chick : how it disappears in the rabbit 

 we shall see in connexion with the urogenital system (13 

 below). The urinary bladder of the frog, which becomes the 

 allantois in the chick and rabbit, arises from this cloaca 

 ventrally ; the allantois is entirely lost in the adult fowl, 

 but the base of it forms the urinary bladder in the rabbit. 



5. The Early Circulation. Fig. 143, A, shows the primitive 

 condition of the main blood-vessels that obtains (with slight 

 modifications) in all three types. It corresponds in a 

 general way with the arrangement in Amphioxus, with the 

 addition of anterior and posterior cardinal and Cuvierian 

 veins and of arteries to the corresponding regions. One strik- 

 ing difference seen in chick and rabbit, though not in the 

 frog, is due to the presence of a yolk-sac and the im- 

 perfect closing-in of the ventral side of the mesenteron 

 and body-wall. This renders a median sub-intestinal vein 

 at first impossible, and instead we have a pair of such veins 

 as near on either side the middle line as possible, and which 

 unite into one as soon as the pinching of the embryo off 

 the yolk enables them to do so. In the rabbit, even the 

 heart has to begin as a paired structure (the forward 

 continuation of this pair of vessels), but these unite in the 



