REPRODUCTIVE SYSTEM OF THE PIGEON. 515 



aorta by renal arteries, and the filtered blood leaves them 

 by renal veins which unite, as already explained, with 

 femorals and renal portals to form the iliac veins, or, we 

 may almost say, the inferior vena cava. But there can be 

 little doubt that the kidney also receives venous blood 

 from the sciatic and other posterior veins. In other words 

 there is a renal portal system. 



The waste-products, consisting for the most part of urates, 

 pass in semi-solid form down the ureters into the median 

 compartment of the cloaca. 



In front of each kidney, at the base of the iliac vein, 

 there lies a supra-renal body of unknown functional signi- 

 ficance. 



Reproductive System. — The testes lie in front of the kidneys. 

 They increase in size before the breeding season. 



The spermatozoa pass from each testis into a vas deferens, 

 which lies to the outside of the corresponding ureter. The 

 vasa deferentia, slightly convoluted when full of sperms, open 

 separately into the median compartment of the cloaca. 



In the adult pigeon, and in most birds, there is only one 

 ovary ; that of the right side atrophies very early in life. 

 We do not know the reason of this. The right oviduct is 

 represented by a small rudiment close to the cloaca. 



The ovary is covered with follicles containing ova at 

 various stages of ripeness. As these ova become dilated 

 with yolk and otherwise mature they burst from the ovary, 

 and are caught by the dilated end of the oviduct. The 

 first part of the duct is narrow, and there the ova may be 

 fertilised ; the second part is wide and glandular, secreting 

 the white of egg ; in the third region, which is muscular and 

 glandular, the shell of the egg is made. In sexual union 

 the cloaca of the male is closely apposed to that of the 

 female; only in a few cases (in ducks and some other 

 aquatic birds and in the Ratitee), is there a special copula- 

 tory organ. 



Development of the Chick. 

 (Summarised from Foster and Balfour's " Elements of Embryology." 



The ovum of the fowl, when it has burst from the ovary, and has been 

 caught_ by the dilated end of the oviduct, is a large spherical body, 

 consisting for the most part of yolk, but bearing at one region a small 



