414 MANUAL OF ELEMENTAR Y ZOOLOGY 



The kidneys are metanephric (p. 445 )• They lie in the 

 back under the sacrum as a pair of three-lobed 

 R^roduotfve 11 bodies - From the hinder lobe of each a ureter 

 organs"" '" runs back to the cloaca. There is no bladder. 

 The sexes are, of course, separate. The testes 

 lie in front of the kidneys. From each of them the vas 

 deferens, corresponding to the Wolffian duct of the dogfish 

 and frog, runs back on the outer side of the ureter to end 

 in a small swelling or seminal vesicle which opens into the 

 cloaca. When it is full of ripe sperm the vas deferens is 

 slightly convoluted. There being no penis, the sperm is 

 passed in coition by the cloaca of the male being closely 

 apposed to that of the female. The adult pigeon has only 

 one ovary, that of the right side having atrophied early in 

 life. The right oviduct also atrophies, but a small vestige 

 remains, attached to the cloaca. Trie ovary is covered 

 with follicles which contain ova in various stages of ripe- 

 ness. The oviduct is a wide, twisted tube, thin-walled in 

 front and thick behind, opening into the body cavity by 

 a long funnel just behind the ovary. When the ova are 

 ripe they are shed into the body cavity and immediately 

 caught by the opening of the oviduct. Each ovum is a 

 large, round, yellow body corresponding to the " yolk " of 

 the egg. It is a single gigantic cell, so full of yolk that 

 the protoplasm is practically restricted to a small patch at 

 one side, containing the nucleus. It is fertilised in the thin 

 region of the oviduct, coated with white of egg in the first part 

 of the thick region, and provided with a double membrane 

 and a porous chalky shell in the hinder part. The eggs 

 are hatched by the warmth of the body of the parents, who 

 sit upon them in turns. The young, which emerge after 

 sixteen days, are provided with a scanty yellow down and, 

 unlike young chickens, are at first quite helpless, with 

 closed eyelids. They are fed by their parents with a 

 creamy fluid known as " pigeon's milk " formed by the 

 breaking down of the epithelium of the crop. They are 

 fledged at the end of three weeks, and after a few days' educa- 

 tion in flight by their parents go out into the world for 

 themselves. 



The blood has a temperature of 42° C, which is higher 

 than that of mammals. This fact is no doubt connected 



