AVES 



407 



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. 



FIG. 207. 



g.w. 



FIG. 207. Diagram of a longitudinal section of the embryo of a fowl, without the amnion and 

 allantois. Ectodermal boundaries are in continuous lines, the entodermal and mesodermal are in 

 broken lines: the entodermal of short dashes, the mesodermal of long, b, brain; b.iv., body wall; 

 c.C., central canal of spinal cord; co., coelom; g, gut; g.w., wall of gut; s.c. t spinal cord; y.s., yolk sac. 



Questions on the figure. What is the relation of the yolk sac to the digestive 

 cavity ? Which of the embryonic layers surrounds it ? In what way is the abun- 

 dant yolk in the yolk sac brought into the circulation of the embryo (see reference 

 texts) ? 



440. Reproduction and Development. Reference has al- 

 ready 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 



