GENERAL ZOOLOGY 



tive egg membrane prevents drying. There is a much larger amount of yolk 

 than in the eggs of amphibians, and the time of hatching in reptiles is delayed 

 until the young have reached the stage of juveniles able to care for themselves. 

 Some reptiles have been modified from this state and have become ovoviviparous; 

 that is, zygotes are retained within the body of the female, and development 

 proceeds in a specialized portion of the female reproductive tract. Compari- 

 son of mammalian development with that of reptiles makes it evident that the 

 viviparous development of most mammals has evolved from oviparity like that 

 of reptiles. Indeed, the members of one group of primitive mammals, including 

 the duckbill and the spiny anteater, still lay eggs. 



The earliest known reptiles are from the Carboniferous (Fig. 18.17). These 

 primitive types, or "stem reptiles," from which the later members of the class 

 seem to have arisen, are so like amphibians that they might be classified in 

 either group. The short limbs extend laterally rather than downward from 

 the body, and it is evident that in life the body still rested upon the ground 

 as it does in amphibians. The diversification of reptiles from the stem forms 

 began in the Permian; as the Amphibia declined, the Reptilia became the 

 predominant land vertebrates during the Mesozoic. Among the lines that 

 flourished, only to become extinct with the rise of the Mammalia in later 

 times, were the dinosaurs (see Figs. 18.19, 18.25, 18.27), the most diversified 

 group of terrestrial vertebrates that has ever lived. Related forms were the 

 ichthyosaurs and mosasaurs which invaded the sea (Fig. 18.18); the pterosaurs, 

 flying reptiles distinct from the line giving rise to the birds (Fig. 18.19); 

 reptiles ancestral to the birds themselves (see Fig. 18.25); and reptiles 

 ancestral to mammals (see Fig. 18.29). The existing reptiles are remnants of 

 a mighty race, of which at one time the dinosaurs ruled the land, the 

 ichthyosaurs the water, and pterosaurs the air. We can only guess at the 

 factors involved in the decline of such a group. Reptiles flourished as 

 animals that were obviously better suited to terrestrial life than amphibians. 



566 



