THE VERTEBRATES 449 



from the yolk and later from the " white/' (2) obtaining 

 oxygen from air which filters in through the pores of the 

 shell, and (3) the discharge of carbon dioxide made by the 

 developing embryo. The importance of the two later pro- 

 cesses, which considered together constitute the respiration 

 of the egg, is shown by the fact that the embryo dies from 

 asphyxiation (lack of oxygen and excess of carbon dioxide) 

 if the pores of the shell become filled, as when coated with 

 albumen from another egg accidentally broken. Hence, 

 poultry keepers must take great care to keep eggs intended 

 for hatching clean before and during incubation. 



The time for incubation varies. Hens' eggs hatch in three 

 weeks, ducks' eggs in four weeks, and other birds have either 

 shorter or longer incubation. No one knows why the egg 

 of one species of bird requires more time than does that of 

 others. 



366. Reptile Development. The embryology of many 

 snakes, alligators, and turtles is very similar to that of birds, 

 except that incubation is due to the sun's heat ancf not to 

 brooding by parents. Some snakes and lizards are viviparous, 

 the eggs being retained in the oviducts until fully developed 

 into young animals which are then ejected by muscular con- 

 traction of the oviducts. The eggs of these viviparous 

 reptiles are similar to the oviparous ones and contain much 

 stored food to be used by the developing embryos. 



367. Mammalian Development. We have already (in 

 63) noted the advantages of internal or viviparous develop- 

 ment over the external or oviparous method ; and that while 

 cases of internal development occur among animals of many 

 groups, it is in the mammals that we find vivipary universal, 

 with the one exception of the Australian duck-bill, which 

 lays eggs. 



In adaptation to viviparous development the oviducts of 

 the mammals have been modified into a sac-like structure in 

 whose cavity eggs lie during their embryonic development. 



2G 



