IV. VERTEBRATA: REPTILIA. 593 



The venous character of the left aortic arch and the incomplete 

 ventricular septum (or presence of foramen Panizzae) prevent a 

 complete separation of systemic and pulmonary circulations. In 

 the turtles a third element enters, the persistence of a ductus 

 Botalli (as in Urodeles, fig. 580, II, dB). 



To the foregoing adaptations to a terrestrial life may be added 

 indications of higher development. The brain shows two advances. 

 The cerebellum, especially in turtles and alligators, has be- 

 come large, and the cerebrum grows dorsally and backwards 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 eye 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 f enestra rotunda, 

 places the tympanic cavity and the labyrinth in close relations. 



In the excretory system amniote characters prevail. The 

 Wolffian body with its duct is functional in the embryo. Later 

 there arises behind it the permanent kidney (metanephros) with 

 the ureter, while the embryonic structures disappear with the ex- 

 ception of 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). This latter is lacking in snakes and 

 crocodiles. 



Almost all reptiles lay eggs; only in the Squamata (some snakes 

 and lizards) are viviparous or ovoviviparous forms present. The 

 eggs much resemble those of birds, in that the large yolk is sur- 

 rounded with a layer of albumen and enclosed in a fibrous, often 

 calcified shell. 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. From these relations it follows 

 that internal impregnation must occur; the eggs undergo a discoidal 

 (meroblastic) segmentation. Copulatory organs to accomplish 

 this internal fertilization occur, and these are of classificatory im- 

 portance, 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- 



