xiv. 8 REPRODUCTION 381 



traversing this region they are carried into a groove along each penis. 

 In snakes the sperms may survive within the female for long periods, 

 and instances are known of isolated individuals laying fertile eggs 

 after months, sometimes even years, in captivity. 



Some of the most serious difficulties in the colonization of the land 

 are concerned with reproduction, and these problems have been 

 largely solved in the reptiles, allowing the animals to reproduce with- 

 out returning, as many amphibia must do, to the water. 



The eggs of oviparous reptiles are always laid on land. They there- 

 fore require a firm physical support and protection against desic- 

 cation, as well as an adequate supply of food and special means of 

 gaseous exchange and storage of waste products. These requirements 

 are met by the development of a shell, secreted by the walls of the 

 oviduct and often hardened by lime impregnation, by the formation 

 of special embryonic membranes, the amnion and allantois, and by 

 the provision of a large quantity of yolk enclosed in a bag, the yolk- 

 sac. The method of embryonic cleavage is affected by the great 

 amount of yolk, and as in birds is only partial. An albumen or egg- 

 white layer is present in the eggs of crocodiles and tortoises and pre- 

 sumably serves as a reservoir of water; in the eggs of lizards and 

 snakes, however, the albuminous layer is poorly developed or absent. 



The formation of the amnion and allantois is one of the most 

 remarkable features of the development of reptiles ; it is characteristic 

 of all higher vertebrates, distinguishing them sharply from the lower 

 types. The amnion is developed from folds, which cover the embryo 

 and enclose a sac filled with fluid, where development can proceed 

 in the absence of the pond that was necessary for the earlier verte- 

 brates. The allantois began as an enlarged bladder, serving for the 

 reception of the waste products during the life within the shell. Coming 

 close to the surface and fusing with the chorion, it then becomes the 

 vehicle for the transport of oxygen to the embryo. 



The evolution of eggs and embryonic membranes of the kind de- 

 scribed must have been an event of critical importance in tetrapod 

 history. Romer has suggested that this advance took place under 

 climatic conditions of alternate drought and flooding, so that eggs 

 laid above the high-water mark had the best chance of survival. Since 

 many of the early reptiles are thought to have spent much of their 

 time in the water, it is possible that the egg preceded the adult in the 

 process of adaptation to terrestrial life. 



Most reptiles lay their eggs, but in many lizards and snakes these 

 are retained within the oviduct until the young are ready or nearly 



