382 REPTILES xiv. 8- 



ready to hatch (e.g. Lacerta vivipara, Anguis fragilis, Vipera berus). 

 This method of reproduction is termed ovoviviparous ; in forms that 

 practise it the eggshell is reduced to a thin membrane or is lost 

 altogether. In some species (e.g. certain skinks and other lizards, 

 sea-snakes) a placenta is developed from the chorio-allantois or the 

 yolk-sac or both. The placenta may, as in Lacerta vivipara, serve only 

 for the transfer of water and gases, but in the more advanced forms it 

 probably provides a means of transport for food (supplementing the 

 yolk) and excretory products. 



Young born alive are perhaps less susceptible to the hazards of 

 weather than those left to hatch in the sun or among rotting vegeta- 

 tion, and it is interesting that most, if not all, of the few reptiles that 

 live in places where the climate is really severe are ovoviviparous. 



Young reptiles have special devices to assist their escape from the 

 egg. In Sphenodon, Chelonia, and Crocodilia, as in birds, there is a 

 horny epidermal egg-breaker on top of the snout tip, called the egg- 

 caruncle. In the Squamata, a true egg-tooth, projecting from the front 

 of the upper jaw, has the same function. The egg-tooth is present, 

 though sometimes rudimentary, in ovoviviparous forms. 



Some reptiles make a simple nest but the group is not noted for 

 maternal care, usually abandoning their new-laid eggs or newborn 

 young. There are, however, some exceptions to this; female pythons 

 and certain other snakes and lizards brood their eggs, and female 

 alligators are said to guard their nests. 



Many reptiles exhibit well-marked courtship and display pheno- 

 mena during the breeding-season, the males fighting and displaying, 

 either to intimidate each other or to evoke a suitable response from 

 the female. This is particularly striking in certain lizards, notably 

 those of the iguanid and agamid groups, where the males are often 

 brightly coloured and may be adorned with crests and distensible fans 

 under the throat. In these lizards bobbing movements of the head 

 and front part of the body, often accompanied by colour change, form 

 an important part of the display. As in birds, courtship may be associ- 

 ated with territory, a male holding an area of ground on which females, 

 but not rival males, are tolerated. Breeding behaviour and sexual 

 coloration are, of course, under the control of the endocrine system, 

 especially the anterior pituitary and the gonads, and may be modified 

 by castration. The onset of the breeding season is also influenced by 

 climatic conditions; most reptiles breed only once or twice a year, 

 but a few species living in warm stable climates may breed at intervals 

 nearly all the year round. 



