330 ELEMENTARY BIOLOGY 



of living sperms for two or three years, or even much longer, 

 and to force these out of the receptacle from time to time as 

 she produces new eggs. Even the insects that normally lay 

 their eggs in the water — as the mosquitoes — have fertilization 

 take place within the body of the mother. 



383. Reproduction in vertebrates. Among all the backboned 

 animals, above the amphibians, fertilization takes place within 

 the body of the mother. The eggs begin to develop immedi- 

 ately after fertilization and are retained within the parent's 

 body for a longer or a shorter period. Here they are not only 

 protected against possible injury by enemies, but they are 

 nourished and supplied with moisture and, in some cases, 

 kept warm. 



The degree to which the new organism is dependent upon 

 the parent during the early stages in its development varies 

 considerably. Among the reptiles — for example, some tor- 

 toises and alligators — the developing egg becomes enveloped in 

 a mass of food material on its way out of the mother's body 

 and is then supplied with a horny shell. The egg is then 

 deposited in the sand, where it hatches under the heat of the 

 sun. In certain lizards, however, the eggs hatch within the 

 body of the mother, and the young leave her body fully formed. 



Among the birds the fertilized egg becomes covered with a 

 large quantity of food material (yolk and egg albumen), and the 

 whole mass becomes surrounded by a limy shell. Nearly all 

 birds protect their eggs, and they also supply the heat necessary 

 for the hatching of the young. 



Among the mammals the development of the egg takes place 

 entirely within the body of the parent. The new organism is 

 cared for not only until it leaves the body of the parent but 

 for a comparatively long period after it is born. The length of 

 this period varies almost directly with the level of the family 

 of animals in the scale of development. 



