WEIGHTS OF ORGANS IN UNDERFED YOUNG RATS 123 



beginning at the age of three weeks there is an increase in the 

 lengths of both body and tail. The latter increases more rapidly, 

 however, so that it tends to assume the tail-ratio found in normal 

 rats of corresponding age. This indicates that the skeleton not 

 only continues to grow (though at a reduced rate) while the body- 

 weight is held constant, but also tends to grow in a yiormal manner, 

 so as to produce the normal ratio of tail-length and body-length. 

 The preceding paragraphs have shown that the increased growth 

 of skeleton affects the ligamentous as well as the cartilaginous 

 and bony components, and that the chemical composition (per- 

 centages of water and dry substance) also changes in a manner 

 tending to assume the normal. 



The question naturally arises as to whether the skeletal growth 

 during constant body-weight is merely a growth in mass, or is 

 associated with the normal process of differentiation. During 

 the present investigation a few observations have been made upon 

 the development of the normal skeleton, indicating some of the 

 more obvious changes during the age-periods of the rats under 

 experiment, especially between the ages of three and ten weeks. 

 While a detailed study of the developmental changes in the skele- 

 ton is reserved for a separate paper, some preliminary con- 

 clusions may be noted here. 



In skeletons of rats held at constant body-weight from the age 

 of three to the age of ten weeks, the appearance and fusion of 

 certain epiphyses may be noted as in the normal animal during 

 this period, although in most cases the process appears to be 

 retarded somewhat. The following examples may be cited. 

 In the normal skeleton at three weeks of age, the epiphyses at 

 the ends of the vertebral bodies have not appeared; the epiphysis 

 at the lower end of the humerus is well developed, but not fused 

 with the shaft; the maxilla and mandible present each two molars 

 (on each side), with no visible trace of a third. In the normal 

 skeleton at ten weeks, the epiphyses at the ends of the verte- 

 bral bodies have appeared, and most of them have united with 

 the corresponding bones; the epiphysis at the lower end of the 

 humerus is firmly fused with the shaft ; well developed third 

 molar teeth have appeared, both in the maxilla and in the mandi- 



