246 FROM FISH TO PHILOSOPHER 



It is not known how many truly independent reabsorp- 

 tive systems are involved in the vertebrate kidney (or 

 even in man), but the number must be very large. In 

 tubular excretion, however, only two transport systems 

 have been clearly identified; each of these operates on 

 a group of chemically related compounds, one group 

 on nonmetaboKzable aromatic acid residues containing a 

 free acetyl group (— CH2 COOH), the other on non- 

 metabolizable quaternary ammonium bases {62}. 



V. THE ELASMOBRANCHS 



63a. Cohen, J. J., M. A. Krupp, and C. A. Chidsey m. 

 Renal conservation of trimethylamine oxide in the 

 spiny dogfish, Sqiudus acanthias. American Journal 

 of Physiology, 194: 229. 1958. 



63b. Cohen, J. J., M. A, Krupp, C. A. Chidsey ni, and 

 C. I. BiLTZ. Effect of trimethylamine and its homo- 

 logues on renal conservation of trimethylamine oxide 

 in the spiny dogfish, Squulus acanthias. American 

 Journal of Physiology, 196: 93. 1959. 



63c. Grafflin, a. L., and R. G. Gould, Jr. Renal 

 fimction in marine teleosts: II. The nitrogenous con- 

 stituents of the urine of sculpin and flounder, with 

 particular reference to trimethylamine oxide. Bio- 

 logical Bulletin, 70: 16. 1936. 



64. Kempton, R. T. Studies on the elasmobranch kid- 

 ney: I. The structure of the renal tubule of the spiny 

 dogfish (Squalus acanthias ). /owrnflZ of Morphology, 

 73: 247. 1943. 



65. Krogh, a. Osmotic Regulation in Aquatic Animals. 

 Cambridge University Press, 1939. 



66. Moy-Thomas, J. H, Palaeozoic Fishes. Chemical 

 Pubhshing Co., New York, 1939. 



67. Needham, J. Contributions of chemical physiology 

 to the problem of reversibility in evolution. Biologi- 

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