DEVELOPMENT OF RANA 185 



beneath them. Part of the cavity of this diverticulum becomes 

 the gall-bladder, and the open connexion with the rest of the 

 gut persists as the bile-duct. Close to this point, the pancreas 

 arises as a number (three) of outgrowths, which remain con- 

 nected with the gut by the pancreatic duct. 



The cavity of the intestine is still small owing to the 

 presence of the yolk-cells. After hatching, this yolk becomes 

 absorbed and the intestine elongates very much, becoming 

 coiled like a watchspring. Behind the intestine is the region 

 of the gut which will become the rectum and cloaca. A 

 downgrowth from the latter gives rise to the urinary bladder. 



During this time, the right and left splanchnoccelic cavities 

 have applied their outer for somatic) layer to the body- wall, 

 and their inner (or splanchnic) layer to the endoderm of the 

 gut and all its derivatives. Ventrally, most of the membranes 

 forming the separation between the right and left splanchno- 

 ccelic cavities break down ; but dorsally these walls persist 

 forming the dorsal mesentery. This mesentery is composed 

 of two closely apposed layers of ccelomic epithelium spreading 

 round the gut and suspending it. It may be noticed, therefore, 

 that the gut is not strictly in the ccelomic cavity at all ; it merely 

 hangs in a fold of ccelomic epithelium which bulges into the 

 ccelomic cavity. From the cells of this splanchnic layer are 

 developed the smooth muscles of the stomach, intestine, and 

 bladder. 



Blood-vessels. — Beneath the floor of the gut, and between 

 it and the underlying splanchnic layer of ccelomic epithelium, 

 there are some scattered mesoderm-cells which become arranged 

 in the form of a tube, or subintestinal vessel. In the region 

 of the pharynx, this tube forms the endothelial lining of the 

 heart. The ccelomic epithelium (splanchnic layer) surrounds 

 this tube and suspends it as it were in a little mesentery of its 

 own from the floor of the pharynx (the dorsal mesocardium). 

 The musculature of the wall of the heart is derived from this 

 layer of ccelomic epithelium, and that part of the splanchnocoel 

 in which the heart finds itself is now called the pericardium. 

 Later on, the various parts of the heart are differentiated. 

 Posteriorly, the heart is continuous with two tubes, the vitelline 



