THE TRANSITION TO LAND 



383 



a great part of their time in the water, but they are typical land animals 

 and some reptiles inhabit some of the most arid regions of the world. 

 The water egg of fish or amphibians consists of an outer membrane 

 containing the developing embryo attached to a yolk sac for nourishment. 

 The land egg of reptiles or birds must have a thick, protective shell on 

 the outside. This shell may be tough and leather-like, as in the reptiles, 

 or brittle as in the birds. Beneath the shell another membrane is devel- 



Paintings by C. R. Knight in Chicago Natural History Museum 



Fig. 26.5. Reptiles of the past. Modern reptiles are but a small remnant of the^ great 

 number that once dominated the earth. These pictures show four of the prehistoric 

 forms in habitat scenes as worked out from fossil remains. From left to right and top 

 to bottom they are : Plesiosaurus, swimming lizards ; Stegosaurus, the armored dino- 

 saur ; Protoceratops, one of the smaller dinosaurs; and Brontosaurus, the thunder 



lizard, largest of the dinosaurs. 



oped, the allantois, which is necessary for respiration. This membrane 

 is supplied with many blood vessels from the embryo and the circulating 

 blood absorbs oxygen through the porous shell and releases carbon di- 

 oxide. The embryo is surrounded by another membrane, the amnion, 

 which contains the amniotic fluid, in which the embryo floats. Thus, it 

 is evident that the embryos of land animals develop in a liquid medium 

 after all, even though the egg remains on the dry land. The exact rela- 

 tionship of these membranes is shown in Fig. 26.4. Reptile's eggs do 

 not need to be incubated, since they are cold-blooded animals, so they 

 are usually buried in the ground and left to hatch. 



