634 EXCRETING ORGANS. 



papillae, and, there being no urinary bladder in birds, the 

 urine, containing a large proportion of urea with little aqueous 

 constituents, is mixed with the other excretions in the cloaca. 

 The openings of the ureters thus preserve the same relative 

 situation as in reptiles and lower vertebrata. But in the 

 ostrich, which presents so many other affinities with the 

 mammalia, the two ureters open at the lower margin of 

 the large cloacal cavity, which allows the secretion to ac- 

 cumulate as in a distinct bladder. The bladder, indeed, 

 in its most normal form, is only a development of the 

 cloacal part of the intestine, and the want of a urinary 

 bladder in adult birds, is due to the extent of oblitera- 

 tion of the allantois and urachus originally continued 

 from their cloaca. The minute cylindrical uriniferous tubuli, 

 much larger than the capillary blood-vessels, and directed in 

 a pinnate manner to the surface of the renal lobules, leave 

 perceptible interlobular spaces for the blood-vessels, as in 

 other glands, and the small sanguineous vesicles, or corpus- 

 cula Malpighi, are seen on these vessels in the tissue of the 

 organ, as in mammalia. The larger branches of the urinife- 

 rous ducts unite to open by prominent papillae into the 

 ureters, and small calices were already detected by Ferrein 

 in the pigeon, and are seen in the kidneys of the cassowary, 

 the falcon, the pintado, and other birds. 



The kidneys of birds first appear in the embryo as a soft 

 transparent, almost homogeneous mass, in which the convo- 

 luted and foliated structure is gradually evolved, and the 

 extremities of the uriniferous tubes which compose the 

 lobules become perceptible in the periphery of the vascular 

 blastema. For some days after escaping from the egg, in 

 the larger birds, the delicate convolutions on the surface of 

 the renal lobes, and the elegant pinnate arrangements of the 

 ultimate tubuli, are beautifully manifested to the naked eye, 

 by means of the natural secretion of white inspissated urea 

 which fills and distends all these parts ; but by immersion 

 in alcohol, this beautiful appearance is soon effaced, by the 

 uniform whitening of the whole surface. These organs are 

 preceded in their development by the two elongated follicular 

 glands, the corpora Wolffiana, the ducts of which proceed 

 likewise along with the ureters to the cloaca. The deciduous 

 corpora Wolffiana, composed of simple tortuous follicles pro- 

 ceeding transversely from their common marginal duct, and 



