ANATOMY OF THE FROG 43 



air that an exchange of the gaseous constituents of blood and 

 air is readily effected, the blood taking up oxygen and giving 

 in return carbonic acid gas. The lungs, as appears from 

 this description, are offsets of the alimentary tract, and they 

 are formed, in the course of the growth of the tadpole into 

 the frog, as an outgrowth from the ventral wall of the gut in 

 the throat region, the outgrowth soon becoming bi-lobed to 

 form the pair of lung sacs. 



The other abdominal viscera of the frog are the spleen, 

 the kidneys with the adrenal bodies, and the reproductive 

 organs. 



The spleen is a small, rounded-oval, dark-red body lying in 

 the mesentery near the commencement of the large intestine. 

 It belongs to the lymphatic system, and has no duct. 



The kidneys are two elongated, flattened, dark-red bodies 

 lying one on each side of the vertebral column towards the 

 hinder end of the abdominal cavity. They lie in the large 

 lymph space formed behind the peritoneal lining of this part 

 of the coelom, and, as they are covered over by the peritoneum, 

 they are, like the gut and other organs which we have studied, 

 outside the coelom. Each kidney has a straight border turned 

 towards the vertebral column, and a convex outer margin. 

 The kidney consists of a number of convoluted tubules, the 

 uriniferous tubules, bound together by fibrous connective 

 tissue, and abundantly supplied with blood-vessels. The 

 tubules open into collecting ducts, which unite together to 

 form a single duct, the ureter, leading to the cloaca. The 

 ureters are short, straight tubes which arise from the outer 

 edges of the kidneys, and run backward, opening into the 

 cloaca on its dorsal side, opposite the opening of the bladder. 

 In the male frog each ureter presents a spindle-shaped dilata- 

 tion on its outer side, the vesicula seminalis. 



The adrenal bodies are found on the ventral sides of the 

 kidneys, near their outer borders. They are small yellow 

 patches, of somewhat doubtful function and significance. 



The reproductive organs consist of the testes and their 

 ducts in the male, the ovaries and their ducts in the female. 

 The testes are a pair of ovoid pale-yellow bodies attached to 

 the dorsal wall of the body cavity by a fold of the peritoneum, 

 the mesorchium. They lie on the ventral sides of the kidneys, 

 to which they are attached by a variable number of very fine 



