TECHNICAL NOTES AND BIBLIOGRAPHY 267 



into the blood and must be excreted in the urine. These 

 divalent ions, with phosphate and some additional sul- 

 fate derived from protein metabolism, comprise the ma- 

 jor inorganic constituents in the urine, which normally 

 is almost sodium chloride free {124}. So far as water 

 balance is concerned, these divalent ions involve so 

 much waste motion because they require for their ex- 

 cretion more water than accompanies them into the body 

 in the ingested sea water. 



This unique urinary mixture in the marine teleosts 

 poses another physiological problem for the animal, be- 

 cause calcium and magnesium salts, when brought to 

 neutrahty, precipitate as the corresponding insoluble 

 hydroxides or oxides; and in neutral solution, in the pres- 

 ence of phosphate ions, magnesium forms the insoluble 

 magnesium acid phosphate {130}. Consequently, were 

 the mrine to shift from an acid to neutral or alkaline re- 

 action, precipitation of these salts would lead to obstruc- 

 tion of the renal tubules. This consideration is undoubt- 

 edly related to the fact that the urine cannot be rendered 

 alkaline in the dogfish {131} or marine teleost by the 

 administration of sodium bicarbonate and other methods 

 which are efi^ective in other animals. 



Water is, of course, also excreted by the aglomerular 

 kidney, but it may be assumed that this water is drawn 

 passively into the tubular urine by osmosis as the urinary 

 solutes are excreted. The observations of Keosian {127} 

 on the osmotic pressure of the fluid in in vitro cultures of 

 the chick mesonephros support the latter interpretation, 

 as do those of Bieter {101, 102, 103} on the diuresis pro- 

 duced by various salts in the toadfish. We believe the 

 fact that the secretion pressure during salt diuresis in 

 the aglomerular kidney can exceed the pressure in the 

 dorsal aorta may be interpreted as an osmotic phenome- 

 non, rather than as evidence for the specific secretion of 

 water. In this interpretation, the fact that during immer- 

 sion in diluted sea water the inulin U/P ratio in the 



