CHARACTERS OF AVES. 469 



Other class of the Vertebrata. This result is also to a consider- 

 able extent conditioned by the non-conducting nature of the 

 combined down and feathers which form the integumentary 

 covering of Birds. 



The urinary organs of Birds consist of two elongated kid- 

 neys and two ureters, but there is no urinary bladder. The 

 ureters open into the cloaca, or into a small urogenital sac 

 which communicates with the cloaca. 



As regards the reproductive organs^ the males have two testes 

 placed above the upper extremities of the kidneys, and their 

 efferent ducts {;vasa defermtid) open into the cloaca alongside 

 of the ureters. A male organ {penis) may or may not be pre- 

 sent, but there is no perfect urethra. The female bird, as a 

 general rule, is provided with only one ovary and oviduct — 

 that of the left side' — the corresponding organs of the right side 

 being rudimentary or absent. The oviduct is very long and 

 tortuous, and the egg, during its passage through it, receives 

 the albuminous covering which serves for the nutrition of the 

 embryo, and which is known as the " white " of the egg. The 

 lower portion of the oviduct is dilated, and the egg receives 

 here the calcareous covering which constitutes the "shell." 

 Finally,. the oviduct debouches into the cloaca, into which the 

 egg, when ready, is expelled. The further development of the 

 chick is secured by the process of " incubation " or brooding, 

 for which birds are peculiarly adapted, in consequence of the 

 high temperature of their bodies. , 



The development of the ovum belongs to physiology, and 

 does not concern us here. It is sufficient to notice the means 

 by which in many cases the chick is ultimately enabled to escape 

 from the egg. When development has reached a stage at which 

 external life is possible, it is of course necessary for the chick 

 to be liberated from the egg, the shell of which is often ex- 

 tremely hard and resistant. To this end, in very many in- 

 stances, the young bird is provided with a little calcareous 

 knob on the point of the upper mandible, and by means of this 

 it chips out an aperture through the shell. Having effected its 

 purpose, this temporary- appeiKlage then disappears, without 

 leaving a trace behind. , 



The state of the young upon exclusion from the egg is very 

 different in different cases, and in accordance with this. Birds 

 have been divided into the two sections of i\\& Autophagi or 

 Aves pracoces, and the Heterophagi or Aves altrices. In the 

 Autophagi the young bird is able to run about and help itself 

 from the moment of liberation from the egg. In the Hetero- 

 phagi the young are born in a blind and naked state, unable to 



