XXIV] URINO-GENITAL SYSTEM 627 



urodaeum, the upper part of which receives ducts of the kidneys 

 and reproductive organs, while into the dorsal wall of the lower and 

 outer part a glandular pouch of unknown function, called the bursa 

 Fabricii (12, Fig. 313), opens. This becomes smaller and some- 

 times entirely disappears in the adult Bird. 



The structure of the kidneys and reproductive organs is es- 

 sentially the same as in the Reptilia. The meta- 

 oiga n ns. genitai nephros in both sexes is distinctly divided into 

 lobes. The mesonephros is represented by a small 

 lobed epididymis closely adherent to the testes. The suprarenal 

 body (4, Fig. 313) is homologous with the adrenal of Amphibia. 



In most Birds there is no special organ for copulation, the whole 

 end of the cloaca being turned inside out for this purpose, just as in 

 Amphibia and Khynchocephala. That this however is a secondary 

 and not a primary state of affairs is suggested by the existence in 

 Ostriches and some other Birds (Anseriformes) of a long penis on 

 the dorsal wall of the cloaca similar in structure to one of the 

 penes or copulatory sacs of the Lizard. 



There is usually only one functional ovary, the left ; an instance 

 of the economy one observes throughout animated nature, for there 

 is always a tendency when organs become expensive, that is, so 

 large as to be a serious tax on the system, to reduce their number, 

 and the production of eggs of the size of a Bird's is a great drain 

 on the organism. In the case of a few birds of prey it has been 

 recently shown that the right ovary can produce fully developed 

 eggs as well as the left. There are two oviducts, but the right 

 is small and useless. It must be remembered that the true egg 

 formed by the ovary is the yolk; the white and the shell are 

 additions derived from the oviduct. 



The nests which Birds build and their care for the nestlings, 

 whom they in some cases feed at short intervals 

 for about seventeen hours out of the twenty-four, 

 are well known to all. Most also are aware that many birds 

 migrate to other lands as winter sets in. It is less well known that 

 quite as many migrate to lands further north on the approach of 

 spring. Few imagine the enormous distances which are covered 

 by birds on the wing. They constantly pass from the Bermuda 

 Islands to the Bahamas, 600 miles, without a rest. Many species 

 which have their home in North Africa go every spring to North 

 Siberia to build their nests. They fly, when migrating, at such 

 heights in the air as to be quite invisible and attain a pace which 



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