Sauropsida: Class Reptilia 



i«:i 



space between the innermost amnion and the body of the embryo is 

 filled with a watery fluid having a chemical composition resembling 

 that of sea water. The allantois, growing out from the hindgut, con- 

 tains a system of blood-vessels and spreads out in close relation to 

 the inside surface of the porous shell, thus providing for respiratory 

 interchange of gases between the blood and the external air (Fig. 372). 

 The embryo, although that of a land animal, develops in an aquatic 

 medium, the amniotic fluid. This fluid, however, being produced by 

 the embryo itself, cannot be a source of the oxygen necessary for the 

 rapidly growing animal. Therefore gills would be of no use. The early 

 embryo, although immersed in fluid, declares its terrestrial nature by 

 improvising the temporary allantoic "lung" to serve for the breathing 

 of air until emergence from the shell makes it possible to use the 

 internal lungs. 



The reproductive method of reptiles and their complete structural 

 adaptation to terrestrial life open to them all the land surfaces of the 

 earth except so far as temperature may be a restricting factor. Being 

 "cold-blooded" (poikilothermous), reptiles must confine themselves 

 to those regions whose temperatures are not so low as to make animal 

 metabolism impossible. 



Classification 



A classification including all reptiles, present and extinct, requires 

 the defining of 30 or more orders. Reptiles now existing fall into either 

 four or five orders, depending on whether lizards and snakes are 



Fig. 372. Turtle (Chrysemys picta) embryo: enlarged about 3 times. The well- 

 developed eye, the legs, and the beginning of the carapace may be seen. The 

 amnion appears (in optical section) as a very thin pale membrane fitting snugly 

 about the head and body. The nearly spherical yolk-sac is marked by prominent 

 blood-vessels. The fully expanded allantois, seen in the distance, spreads over the 

 inner surface of the shell. (Courtesy, Louis Agassiz: "Embryology of the Turtle," 

 Boston: Little. Brown & Co.) 



