GROWTH OF THE SKELETON 301 



In all these cases this ratio becomes nearly constant at a 

 body length of 125 mm. (body weight, 50 grams), or some time 

 before puberty. 



In the foregoing summary of our observations on the rat, the 

 results to which attention is especially called have been noted. 

 To this summary have been added several comparisons with the 

 data on other mammals, including man. It remains now only 

 to make a few general statements. 



GENERAL CONSIDERATIONS 



Our records do not show just how long the growth of the skele- 

 ton in weight continues in the rat, but from the data at hand we 

 should say it was still growing at 474 days of age, which, accord- 

 ing to our usual computation (Donaldson, '15, p. 6) is equiva- 

 lent to thirty-nine years of human life. The only datum for 

 man with which this can be compared is the linear measurement 

 represented by the stature, which seems to reach its maximum 

 at about twenty-eight years in the human male and twenty-five 

 years in the female. If the increase in the weight of the human 

 skeleton ceases at the time when the increase in stature stops, 

 then it is clear that the growing period for the skeleton of the 

 rat is much longer continued, and this conclusion agrees with 

 our general impressions concerning the growth of this animal. 



It is to be noted, however, that the mature relations among the 

 parts of the skeleton are established for the most part at puberty 

 or earlier, while the weight of the skeleton as a whole, relative 

 to the body weight, tends to decrease slowly as the rat becomes 

 larger. 



During the period between birth and puberty there is, how- 

 ever, an interesting change in the form of the long bones which 

 we have examined in a preliminary way by the study of the 

 weights of a running millimeter at different ages. 



When the changes in the weight-length relations during the 

 growth of the rat are followed by dividing the values in table 26 

 by the corresponding values in table 25, it appears that at 

 maturity the growth in length is about 1.8 times that in diameter, 

 while at puberty it is about 1.7 times, and at weaning about 1.5 



