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Present knowledge of the metabolism-of terrestrial plants and freshwater algae is considerable (cf.

of administration of the isotope, which was given intravenously in the rabbit and intraperitoneally in the cat. SUMMARY 1. The degree of calcification (calcium/nitrogen ratio) of cortical bones in rabbits increases from about the fifth day of life and in the cat from about the ninth day of life. From the birth of kittens until they are 9 days old there is a continuous reduction in the degree of calcification. 2. The calcification of the cancellous bone in rabbits remains approximately constant during the whole of growth, whereas in cats this bone becomes less calcified during growth. 3. The amount of 32p incorporated during the 4 hr. following administration declines throughout the growth of rabbits in both cancellous and cortical bone. In cats there is a maximum incorporation in both types of bone at an age of about 14 days. This maximum is more marked in the cancellous bone. 4. In both cats and rabbits there is a very marked change during growth in the incorporation of 32p in the subperiosteal and endosteal layers of bone. The incorporation of 32p in the former layers declines throughout growth, whereas that in the latter layers reaches its maxiimum in the adult. We wish to thank Mr R. W. Lofthouse and Miss E. B. Morley for excellent technical asstance, and one of us (S.M.W.), would like to thank the Medical Research Council for a research grant received during the earlier part of this work.

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