AVES. 403 



by reference to current works on natural history many inter- 

 esting facts in connection with bird-life. Under the sugges- 

 tive studies a partial list of such topics will be found. In the 

 chapter on Adaptations and in the section on the classification 

 of birds (Ch. VIII, and 432) additional facts have been pre- 

 sented. Much of the time given to the practical studies of the 

 group of birds should be directed to their life and adaptations. 

 These various habits and modes of life have frequently been 

 made the basis of classification : for example, some fly and 

 some do not ; some wade, having long legs ; others swim and 

 have webbed feet; some capture living prey with talons and 

 curved beak ; some scratch and have blunted claws ; some climb 

 and have two digits directed forward and two backward; 

 others perch and have only one toe pointed backward. The 

 resort to such superficial features in classifying birds suggests 

 that the members of the class are more nearly related and more 

 similar among themselves in the fundamental features of struc- 

 ture than is the case with the subdivisions of the other classes 

 of vertebrates. 



431. Reproduction and Development. Reference has 

 already been made to the fact that the right reproductive 

 organs of the female birds are much reduced or wanting. 

 The ovum is always large, containing abundant yolk. When 

 mature it breaks from the ovary, enters the funnel-shaped end 

 of the oviduct and as it passes outward is fertilized. It then 

 receives a layer of albumen, and later is surrounded by a mem- 

 branous covering and by a porous, limy shell, all of which are 

 secreted by the walls of the oviduct. The protoplasm is con- 

 fined to a small germinal disc and segmentation is discoidal, 

 resulting in a blastoderm like that of reptiles. In the newly 

 laid egg cleavage is well advanced. After the egg is laid, 

 cleavage is checked until the necessary temperature for further 

 development is supplied either by the brooding of the parent 

 or by some special device. Owing to the action of gravity 

 on the heavier yolk the living disc is always directed upward, 

 the position most favorable for getting the warmth of the 



