RATE OF KENKWAL OF THE FISH SKELETON 207 



in 1he case of the ('pi])hysis oi' ihc libia and 1o as lit lie as 7 per cent 

 for the diaphysis, while phosphatides extracted from tlie bone tissue 

 are entirely, and possil)ly e\-en several times, renewed in the course 

 of the experiment. 



The experiments mentioned above were carried out wilh fully grown 

 rabbits, as in a growing organism the presence of labelled atoms cannot 

 be interpreted exclusively as the result of a renewal process. It will 

 also be due to the formation o{ additional tissue duiing the experiment. 

 All molecules formed in a growing, labelled organism are, indeed, bound 

 to become labelled. It is of importance, therefore, to carry out experi- 

 ments on the renewal of the skeleton in adult animals. 



The incorporation of labelled phosphate into mineral components 

 of the bone is a very intricate process. Between the uppermost molecular 

 layer of the apatite crystallites in contact with plasma or lymph, an 

 exchange equilibrium can be established almost immediately. This 

 means that the specific activity of the P present in these layers will 

 promptly follow all changes in the specific activity of the plasma phos- 

 phorus. In most experiments with labelled phosphate, the active prepara- 

 tion is administered at the start of the experiment. After subcutaneous 

 injection or administration by mouth, an increase in the plasma acti- 

 vity will take place in the initial phases of the experiment and a decrease 

 throughout the later phases. Thus, the activity of the uppermost layer 

 of the bone apatite is strongest in the early phases of the experiment 

 in which the plasma is strongly active. Crystallites, how^ever, formed 

 in the course of the experiment from an active plasma, will contain 

 comparatively large amounts of ^^P in view of the high phosphorus 

 content of the total crystallites compared with the phosphorus content 

 of the uppermost molecular layer. In view of the stability of the crys- 

 tallites, much of their ^^p content will be conserved and will not follow or 

 follow^ only slowly the changes in the activity of the plasma phosphorus. 



Beside formation of entire crystallites from the labelled plasma we 

 have also to consider the case of partial formation of crystallites. Some 

 molecular layers are dissolved and replaced by layers formed by cris- 

 tallization from labelled plasma. The newly formed layers will be active, 

 but not the layers lying below. These layers will be protected from all 

 action of the labelled plasma and, thus, from renewal. They will form 

 a stable core for the crystallites and so will all crystallites that are not 

 in contact with plasma or lymph. 



Manly and his colleagues (1940), who carried out extensive studies 

 into the uptake of ^^p i^y ^^^ mineral constituents of the bone where the 

 activity was administered at the start of the experiment, estimate the 

 share of labile and stable fractions of the bone tissue by comparing the 

 activity of the blood (not of plasma) P and of the bone mineral of rats. 

 They estimate Vg of the ^ap content to be present after llic lapse of 20 



