XVI. 26 



REPRODUCTION 



475 



whitish granules. There is no urinary bladder in the adult bird. More 

 soluble excretory end substances, such as urea, would reach toxic con- 

 centrations. The glomeruli are much more numerous and smaller 

 than those of mammals. The urinary tubules effect a considerable con- 

 centration of the urine by means of long loops of Henle. The viscous 

 fluid that enters the urodaeum then 

 passes up into the coprodaeum, 

 where further water is abstracted, 

 and the mixed faeces and urinary 

 products are then excreted as the 

 characteristic semi-solid white 

 guano. The water-conservation sys- 

 tem is certainly very effective, and 

 some desert-living birds are said to 

 be able to survive for many weeks 

 without water. In this respect the 

 birds have freed themselves from 

 the original aquatic environment to 

 a remarkable degree. 



RR.OVD 

 OPUR 



OPOVD 

 CL 



Fig. 286. Female reproductive organs 

 of a hen. 



ov. ovary; K. kidneys; f.t. funnel; ovd. 

 oviduct; m.ovd. muscular part of oviduct; 

 OP. OVD. opening of oviduct; ur. ureters; 

 op.ur. opening of right ureter; r.r.ovd. 

 rudimentary right oviduct; cl. cloaca. (From 

 Thompson, Biology of Birds, Sidgwick & 

 Jackson, Ltd.) 



26. Reproductive system 



The testis consists of coiled 

 tubules of the usual type, joining to 

 form a long epididymis and vas 

 deferens, opening into the urodaeum 

 by an erectile papilla that is the only 

 copulatory organ of most birds. 

 During copulation the proctodaea 

 of male and female are everted and 

 pressed together, so that the sperm 

 is ejaculated direct into the female urodaeum and finds its way up 

 the oviduct. A definite penis (and also clitoris) is found in ratites, 

 anseriformes, and a few other birds. The condition of the testis and 

 its ducts varies greatly with the time of year, the weight of the gland 

 being as much as 1,000 times greater in the breeding-season than it is 

 in the non-breeding, when it contains only spermatogonia. 



The provision of material sufficient for the development of a warm- 

 blooded creature is, of course, made possible in birds by the extremely 

 yolky eggs, so large that they allow room for development of only one 

 ovary, nearly always the left (Fig. 286). The right ovary remains present 

 as a rudiment and if the left is destroyed by operation or disease the 



