414 THE PIGEON. PHYLUM CHORDATA 



uric acid is precipitated and the water, with some salts, is saved 

 for the body by reabsorption as in the rectum of the cockroach. 

 There is some tubule excretion from the renal portals. The sexes 

 are 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 sperms the vas deferens is sHghtly 

 convoluted. There being no penis (though some birds, such as ducks, 

 have one made of two incompletely joined halves) the semen 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. 

 The ovary is covered with follicles which contain ova in various 

 stages of ripeness. 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 

 which becomes the ' yolk ' of the egg (Fig. 500). 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 eyehds. They are 

 fed by their parents with a cheesy fluid known as ' pigeon's 

 milk ', rich in protein and fat, which is 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' education in flight by 

 their parents go out into the world for themselves. Birds are 

 remarkable for their elaborate sexual ritual and courtship, 

 which are associated with marked seasonal variations in size 

 of the gonads, and with the activity of the pituitary gland. They 

 also show a very high degree of parental care, the young being 

 fed and defended until they are as large as their parents. 



