DEVELOPMENT OF REPTILES. 495 



of the iguana are laid in tlie hollows of trees. Certain 

 snakes, as the vipers, are viviparous. In many snakes and 

 lizards the development of the embryo goes on in the egg 

 before it leaves the oviduct ; such species are said to be ovo- 

 viviparous, the young being born living. The Eiitmiia 

 sirtalis, or common striped snake, In-ings forth its young 

 alive, and is probably ovovivijjarous rather than viviparous. 



The early phases of the development of the reptiles, in- 

 cluding the origin of the amnion and allantois, is much as 

 in the chick. In the turtle, by the time that the heart has 

 become three - chambered, the vertebras liave reached the 

 root of the tail, the eyes have become entirely enclosed in 

 complete orbits, and the allantois begins to grow. The 

 nostrils may now be recognized as two simple indentations 

 at the end of the head, and at first are not in communica- 

 tion with the mouth, but soon a shallow furrow leads to it. 

 The shield begins to develop by a budding out laterally of 

 the musculo-cutaneous layer along the sides of the body, 

 and by the growth of narrow ribs extending to the edge of 

 the shield. In the oviparous snakes [e.g., Natr'ix torquata) 

 the embryo partially develops before the egg is laid, while 

 the young hatches in two months after the egg is deposited. 

 By this time the amnion is perfected, the head is distinct, 

 and shows the eyeball and ear-sac ; also the maxillary and 

 mandibular processes. The allantois is about as large as the 

 head. The long trunk of the serpent grows in a series of 

 decreasing spirals, and when five or six are formed, the rudi- 

 ments of the liver and the primordial kidneys are discern- 

 ible. At the latter third of emljryonic life the right lung 

 appears as a mere appendage to the beginning of the left. 



The reptiles are essentially tropical and subtropical ani- 

 mals ; they are scarce in north temperate countries, thougli 

 in North America snakes extend north farther than lizards ; 

 in Europe snakes cease at 60° north latitude, and at 6000 

 feet elevation in the Alps ; lizards in Euroi^e sometimes ex- 

 tend farther north than snakes, and ascend to an elevation 

 of 10,000 feet in the Alps. Eeptiles are usually wanting in 

 oceanic islands which possess no indigenous mammals, though 

 lizards are sometimes found on islands where there are 



