SECT. 9] PROTEIN METABOLISM 1115 



In the adult elasmobranch, the urea produced by protein breakdown 

 is retained within the circulation, causing an extraordinarily high 

 blood urea and counterbalancing the osmotic pressure of the saline 

 external medium. This retention of urea as an osmotic device has 

 already been discussed in Section 1-13. As early as 1834 John Davy 

 noted the presence of urea in the uterine cavity of Squalus squatina, a 

 selachian, but did not succeed in finding any in the uterus of Torpedo 

 marmorata. He never detected uric acid in these fishes. Parker and 

 Parker & Liversidge also found abundance of urea, but no protein 

 or uric acid, in the " pseudamniotic Hquid" (i.e. the periviteHine 

 liquid within the egg-cases of the ovoviviparous Mustelus antarcticus, 

 after the disappearance of most of the yolk) . 



Whence comes all this urea? In 1890 von Schroder extirpated the 

 livers of a number of fishes [Scyllium canicula) and observed only a 

 small reduction of the urea content of the muscles (1950 mgm. per 

 cent, before and i860 mgm. per cent, afterwards). The Hver can 

 therefore not be the main source, and probably all the tissues have 

 the power of forming urea from the amino-groups in the food. 

 Arginase appears to be found in great quantities in elasmobranchs; 

 thus Hunter & Dauphinee found the following typical figures: 



Arginase units 



Liver Kidney 

 SclachidL-a {Squalus sucklii) ... ... 319 31 



Teleostean {Sebastodes maliger) ... 29 4 



The arginase of the dogfish liver was twice as active as that from the 

 most active teleostean liver (in the herring, Clupea pallasii) and forty 

 times as active as that from the feeblest (in the tommy-cod, Hexa- 

 grammos stelleri) . It is probable, however, that the contribution of urea 

 made by arginase to the total urea content of the elasmobranch cells 

 would not be large. Hunter & Dauphinee made the interesting 

 observation that the undeveloped eggs of Squalus sucklii contained 

 notable amounts of urea, but no arginase, while both were present 

 in an embryo of 20-5 cm. (see also pp. 1077, 1142 and 1312). 



It may be added that Baglioni found that the selachian heart could 

 not beat properly unless a certain amount of urea was contained in 

 the perfusion fluid. 



The excretion of urea in selachians does not appear to take place 

 wholly through the kidneys. Denis found that only 20 to 50 mgm 



