34° AMPHIBIA xn. 21- 



These last may be neutro-, eosino, or basiphil and are migratory and 

 phagocytic. Thus the white cell picture with which we are familiar in 

 mammals was evidently established a very long time ago. 



There is a globular spleen near the tail of the pancreas. 



The blood of frogs also contains numerous small platelets (thrombo- 

 cytes), which probably break down when in contact with foreign 

 surfaces to produce the thrombin that combines with the fibrinogen 

 of the blood-plasma to produce clotting. 



22. Urinogenital system of Amphibia 



The excretory organs of adult amphibia are always the tubules of 

 the mesonephros. In Rana, where there is a general shortening of 

 the body, these extend over only a small number of segments and the 

 kidneys are compact. In urodeles and the primitive frog Ascaphus 

 the kidneys are elongated and show some evidence of their segmental 

 nature. The mesonephros consists essentially of a series of tubules 

 leading from the nephric funnels to the Wolffian duct. In the frog the 

 funnels do not open into the tubules, however, but into the veins; 

 moreover, they form independently of the rest of the tubule. In the 

 adult there are some 2,000 glomeruli, from each of which a short 

 ciliated tube leads to the proximal convoluted tubule. There follows 

 a second short ciliated region, corresponding in position to the Henle's 

 loop of mammals, and leading to a distal convoluted tubule, which 

 joins the Wolffian duct. 



The blood-supply of the kidney differs from that of mammals in 

 that blood arrives from two distinct sources; the branches of the 

 renal artery run mainly to the glomeruli, those of the renal portal 

 vein to the tubules. This corresponds to the functions now well 

 established for those two parts, namely that the glomerulus filters off 

 water and crystalloids, some of which are then reabsorbed by the 

 tubule. Many details of this process are not clear, however, for 

 instance how the urea concentration in the urine is raised many times 

 above that of the blood. 



The frog, having a moist skin, is presumably in constant danger of 

 osmotic flooding with water when it is submerged, and of desiccation 

 when on land. The flooding is prevented by the efficient functioning 

 of the glomeruli; they allow the frog to excrete as much as one-third 

 of its weight of water per day (man i/50th). The mechanisms for 

 resistance to desiccation are less perfect. There is no long water 

 reabsorbing segment, the part of the tubule corresponding to Henle's 

 loop being short. There is, however, a large cloacal (allantoic) bladder 



