380 REPTILES xiv. 7- 



right and left aortae, and the pulmonary trunk. The opening of the 

 latter lies opposite the right side of the ventricle and receives pre- 

 dominantly venous blood. The left systemic arch opens opposite to 

 the incomplete ventricular septum and receives mixed blood, whereas 

 the right systemic arch opens from the left side of the ventricle and 

 carries almost pure arterial blood. The carotid arteries of both sides 

 arise from the right systemic arch. This 'classical' view of the cir- 

 culation of the blood in the reptilian heart has recently been confirmed 

 by the radiographical studies of Foxon and his colleagues on the green 

 lizard. Variations may be found, however, in different species. 



The venous system is based on the same plan as that of the frog, 

 with pelvic veins receiving blood from the tail and hind legs and 

 returning it to the heart through either an anterior abdominal vein 

 or renal portals, and the inferior vena cava. 



In the urinogenital system is seen another feature characteristic of 

 amniotes, the development of a posterior region, the metanephros, 

 concerned solely with excretion, leaving the mesonephric (Wolffian) 

 duct to function as the vas deferens in the male. There is sometimes 

 an endodermal (allantoic) bladder. 



The waste nitrogen is largely excreted as uric acid and this allows 

 the reabsorption of much of the water in the urodaeum, with precipita- 

 tion of the organic matter as a chalky white mass of urates. The 

 advantage of this method of excretion is that it allows for a greater 

 economy of water than would be possible if the end product was the 

 more soluble urea. There is, however, some variation in the mode of 

 excretion among certain members of the group and this may depend 

 on the manner of life of the species and the necessity for water con- 

 servation. Thus among Chelonia the more aquatic forms (Emys) pro- 

 duce considerable amounts of ammonia and urea, but relatively little 

 uric acid, whereas the last is the main excretory product of the fully 

 terrestrial types, such as the Grecian tortoise (Testudo gracca), which 

 can live under almost desert conditions. 



8. Reproduction of reptiles 



Fertilization has become internal, and in all modern reptiles except 

 Sphenodon special organs of copulation derived from the cloacal wall 

 are developed in the male. In crocodiles and tortoises there is a single 

 median penis, but in lizards and snakes there are a pair of these struc- 

 tures, though only one is inserted at a time. The mechanism of erec- 

 tion involves both muscular action and vascular engorgement. The 

 sperms pass from the vasa deferentia into the urodaeum, and after 



