DEVELOPMENT OF REPTILES. 495. 
of the iguana are laid in the 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 Hutenia 
sirtalis, or common striped snake, brings forth its young: 
alive, and is probably ovoviviparous 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 vertebrae have 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-cutaneons 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 (¢.g., Natrix 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 embryonic 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, though 
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 Europe sometimes ex- 
tend farther north than snakes, and ascend to an elevation 
of 10,000 feet in the Alps. Reptiles are usually wanting in 
oceanic islands which possess no indigenous mammals, though 
lizards are sometimes found on islands where there ara 
