308 REPRODUCTION IN ANIMALS 



hairy animals, the eggs are retained within the body of 

 the mother. 



Nourishment of Young. — Very little food yolk is 

 stored in the eggs of many species of animals, and almost 

 at the outset the young are compelled to forage for 

 themselves. In other species, where the yolk is of greater 

 amount and hatching accordingly is delayed, the struggle 

 for existence begins at a somewhat later period. In 

 either event, however, no parental care is exercised, and 

 the newly hatched offspring are fully equipped for an 

 active independent life. This, in large measure, con- 

 tinues to be the rule in numerous species where the young 

 actually are under the care of the parents. Among the 

 birds, for example, the parents normally supply some 

 nourishment, but in the case of the ostriches, rails, 

 plovers, ducks, quails and several related families, the 

 young very early become independent in this regard. 



In the mammals or hairy animals the eggs are retained 

 in the body of the female, and — with the exception of 

 the curious duckbill and the spiny anteater of the Aus- 

 tralian region — the developing young are provided with 

 special membranes and blood-vessels fitted for the absorp- 

 tion of food substances from the parent. Even after birth 

 they are comparatively helpless, and are further nourished 

 by milk developed in the glands of the mother. 



Secondary Sexual Characters. — Among the lower 

 animals a male usually can be distinguished from a female 

 only by dissection or microscopical examination. The sex 

 of the higher species on the contrary is generally associ- 

 ated with certain structural peculiarities known as second- 

 ary sexual characters. The males, for example, are of 

 larger size than the females, or they may be characterized 

 by highly developed tusks, horns, excessive growth of 

 hair, brilliant plumage, spurs, combs, scent glands, or 

 other organs that are developed in various regions of the 

 body. The female often possesses highly developed 

 organs for the protection and nourishment of the young, 



