VERTEBRATA: AVES. 589 



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 tor- 

 tuous, 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 the chick is ultimately enabled to escape from the 

 egg. When development has reached a stage at which ex- 

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

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

 extremely hard and resistant. To this end the young bird 

 is provided with a^little calcareous knob on the point of the 

 upper mandible, arm by means of this it chips out an aper- 

 ture through the shell, at its blunt end. Having effected its 

 purpose, this temporary appendage 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 the Autophagi or 

 Aves prcecoces, 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 feed themselves, or even to maintain unassisted the neces- 

 sary vital heat. In these birds, therefore, the young require 

 to be brooded over and fed by the parents for a longer or 

 shorter period after exclusion from the egg. 



As regards their nervous system, the brain of Birds is rela- 

 tively larger, especially as regards the size of the cerebrum 

 proper, than the brain of Reptiles, but the chief mass of the 

 latter consists of the corpora striata, and it does not cover 

 the cerebellum. The cerebellum is less developed than in 

 Mammals, the lateral lobes and Pons Varolii being rudiment- 

 ary. The corpus callosum is absent, and the surface of the 

 cerebral hemispheres is devoid of convolutions. 



As regards the organs of the senses, the eyes are always well 



