226 PHILOSOPHY OF ZOOLOGY. 



bones. A ureter proceeds from each, and terminates in 

 the cloaca. The urine has a white chalky appearance, and 

 even in those species which subsist entirely on vegetable 

 matter, it abounds in uric acid. 



The male organs of generation are simple in their struc- 

 ture. The testicles, which are two in number, are situate 

 close to the spine, at the commencement of the kidneys. 

 They vary greatly in size, and are always very small at 

 the beginning of v/inter, enlarging with the approach of 

 spring and the breeding season. They are of a roundish 

 form, or yellowish-white colour, and consist of numerous 

 seminal tubes. Each sends out a spermatic duct, which 

 opens into the cloaca at the summit of a conical process, or, 

 uniting with its neighbour, forms a common duct. The 

 two papillae in which the ducts terminate, are distinctly 

 visible in the common fowl. In the drake, however, the 

 ducts terminate in a long wormshaped tube, which, when 

 at rest, is concealed in the cloaca. 



In the female organs, the ovarium is single, and the ova 

 are inclosed in the yolk-bags, which are supported on short 

 foot-stalks. The largest yolk-bags are placed on the out- 

 side. The oviduct is expanded at its extremity for the 

 reception of the ovum, and, in general, pursues a lengthen- 

 ed tortuous course. It enlarges in size and in the thickness 

 of its walls towards the cloaca. Its first portion nearest 

 the ovarium, is covered internally with numerous papillae, 

 the orifices of the glands which secrete the glaire or white 

 of the egg. The part which follows, is termed the uterus, 

 and exhibits internally numerous longitudinal folds. Here 

 the egg receives the shell. The last portion or vagina, 

 terminates on the left side in the cloaca. 



Birds exhibit very remarkable differences with regard to 

 the size of their eggs, in proportion to the body, and their 

 number. They likewise differ in the method of construct- 



