IV. VERTEBRATA: REPTILIA 525 



one hand with the remaining (left, second) arch and on the other with 

 the right or venous half of the heart. The foramen Panizzoe occurs 

 between this and the right aortic arch. The venous character of the 

 left aortic arch and the incomplete ventricular septum (or presence of 

 foramen Panizzie) prevent a complete separation of systemic and pul- 

 monary circulations. In the turtles a third element enters, the per- 

 sistence of a ductus Botalli (as in Urodeles, fig. 537, 77, dB). 



To the foregoing adaptations to a terrestrial life may be added indica- 

 tions of higher development. The brain shows two advances. The 

 cerebellum, especially in turtles and alligators, has become larger, and 

 the cerebrum grows over the 'twixt brain and forms the temporal lobes 

 of the hemispheres. The parietal organ is developed as nowhere else. 

 In many lizards, it forms an unpaired dorsal e^e lying beneath the skin 

 in the parietal foramen. The paired eyes possess lids (usually upper and 

 lower as well as a nictitating membrane), and frequently (turtles, lizards, 

 and many fossils) a ring of bony plates (sclerotic bones) in the sclera. 

 A new opening in the petrosal, the fenestra rotunda, places the tympanic 

 cavity and the labyrinth in close relations. 



In the excretory system amniote characters prevail. The Wolffian 

 body is functional in the embryo. Later the permanent kidney (metaneph- 

 ros) with the ureter arises behind it, while the embryonic structures dis- 

 appear, except those retained as accessory to the genital apparatus. 

 Thus in the male the vas deferens and epididymis are formed from the 

 Wolffian duct; in the female the Mullerian duct (early lost in the male) 

 becomes the oviduct. Usually the urogenital canals open dorsally in the 

 cloaca, rarely in an elongation of the urinary bladder (Chelonia). The 

 bladder is lacking in snakes and crocodiles. 



Almost all reptiles lay eggs; only in some snakes and lizards are vivi- 

 parous or ovoviviparous forms present. The eggs much resemble those 

 of birds, in that the large yolk is surrounded with a layer of albumen and 

 enclosed in a fibrous, often calcined shell; the eggs undergo a discoidal 

 (meroblastic) segmentation. To open the egg the embryo has an egg 

 tooth on the tip of the snout; this consists of dentine in the Squamata, but 

 elsewhere, as in birds, is horny. As the egg has a shell it follows that 

 internal impregnation must take place. Copulatory organs to accomplish 

 this occur, and these are of classincatory value since they differ in character 

 in the Squamata on the one hand, the turtles and crocodiles on the other. 

 These differences are correlated with differences in the form of cloacal 

 opening and in structure of skull and skin, so that all living species may 

 be divided into two groups, the Squamata, containing the lizards, snakes 

 and Sphenodon, and the Hydrosauria with turtles and crocodiles. This, 



