Reptiles

Reproduction.

Fertilization of the reptilian egg always takes place internally, in contrast to the condition in many Amphibia.

The egg is always large and provided with so large a store of food materials in the form of yolk that the growing embryo, without any additional materials, can hatch in a form capable of fending for itself, and is, indeed, usually a miniature copy of its parents.

This ovum is surrounded by a semi-fluid layer of albumen, and enclosed in a membranous shell which may be calcified as is that of a bird. Usually the egg is laid before development has gone far, but in some cases it is retained within the oviduct until the foetus is ready to be born. These animals, including many lizards and snakes, are thus viviparous. In their case the egg shell is thin, and food materials may pass through it ; indeed, in some cases it is practically absent, and the little lizard secures nourishment from its mother through a special placenta.

Cleavage is meroblastic, resulting only in the formation of a primitive plate of cells. Gastrulation involves an actual invagina tion, resulting in the formation of an archenteron which has both floor and roof. The process is, indeed, similar in principle to that in the Gymnophionan Amphibia. No primitive streak is formed behind the blastopore in Chelonia, Sphenodon, lizards or snakes.

The later development much resembles that of birds or mono tremes. A headfold is formed, followed by tail and lateral folds, which gradually raise the embryo from the yolk and extra embryonic structures. An amnion arises from the extra embryonic somatopleure, as in birds, and an allantois is formed later by a ventral outpushing of the hind gut. It serves not only as a reservoir for the excretory products of the embryo, but also as a respiratory organ. The embryo breaks its way out of the shell by the aid either of an egg-tooth, placed mesially on its nose or of a caruncula on its head.

Further details will be found in the article VERTEBRATE EMBRYOLOGY.

Mode of Life.--No

general statements can be made about the habits of reptiles. If the extinct forms be taken into account it will be found that they have occupied all the habitats which are to-day filled with mammals, except that they are excluded from polar regions by the impossibility of hatching their eggs there, and in extreme cases of achieving a sufficiently high rate of metabolism. The temperature of the body of reptiles, like that of Amphibia, and unlike that of the birds and mammals, is de termined by that of their surroundings, rising when the animal is in a warm place, and sometimes becoming very high in bright sunlight, and falling at night, perhaps to freezing point. The animal's muscular activity always keeps it a very few degrees above the air temperature. Some idea of the variety of habit of

reptiles may be gathered from the section on Locomotion in this article, and further facts from the articles : LIZARD, TORTOISE, CROCODILE, and SNAKES.

Geographical Distribution.

Apart from the limitation im posed by temperature, no general statements can be made about reptilian distribution; any useful account would occupy much space, and involve a discussion of the interrelationships of the families of lizards and snakes, a disputed field.

Geological Distribution.

The earliest bone which has been referred to a reptile is an isolated femur from the Lower Car boniferous of Scotland. Eosauravus, from the Upper Carbon iferous (Middle Coal Measures) of Ohio, is probably a reptile, and Solenodonsaurus, from the top of the Upper Carboniferous of Czechoslovakia is certainly one.

The evolution of the reptiles was rapid, nearly all orders being fully established by the end of Triassic times. Several important orders became extinct at the end of the Trias, but the reptiles were the dominant group of vertebrates to the end of the Meso zoic, when, within a short period though not simultaneously, many orders became extinct, leaving only the four which still survive.

Economic Importance.

Reptiles are of slight importance to man. Poisonous snakes are responsible for many deaths of man and domesticated animals in all tropical and some temperate regions.

The marine turtle, Chelone midas, found in tropical waters of the Atlantic, Indian and Pacific oceans, provides the best of all soups ; several other forms found in fresh water are often eaten, the most familiar of these being the terrapins, of the genus Chrysemys. The eggs of various species are also eaten by un civilized peoples.

The skins of crocodiles, and of certain of the larger lizards and snakes, are tanned and used as leather. This consists only of the cutis, the horny epidermis being removed. This leather is extraordinarily tough and wear resisting, and the presence in it of the papillae which underlie the scales gives it a most attractive surface. The pigment, or at any rate such of it as is melanine, may remain in the leather and give it characteristic patterns. The use of reptilian leathers for ladies' shoes and handbags has become popular and led to the destruction of many of these animals. That of alligators, however, is derived in part from animals bred for the purpose.

Further information about reptiles will be found in the separate articles : CROCODILE, DINOSAURIA, LIZARD, SNAKES, ICHTHYO SAURIA, PLESIOSAURUS, PTERODACTYL, TORTOISE, etc., and in the general articles, EMBRYOLOGY, ANATOMY, PHYSIOLOGY, NEUROL