106 C. M. JACKSON 



portions preserved for microscopic examination (to be con- 

 sidered in a later paper) . 



As heretofore, in calculating the percentage weights, the net 

 body-weight (gross weight less intestinal contents) is taken. 

 The percentage weights of the organs are thus slightly higher 

 than if calculated upon the gross body-weight. 



The averages given m the various tables are the arithmetical 

 means of the corresponding individual observations. In view 

 of the comparatively small number of observations and the 

 known variability, especially of some of the organs (cf. Jackson 

 '13), the data are insufficient for treatment by statistical methods, 

 and the values are therefore only fair approximations. They are, 

 however, sufficiently accurate to show some of the more ob\'ious 

 and important changes in the young animal held at constant 

 body-weight. It is hoped that they may be useful as prelimi- 

 nary observations, which may lead to further and more extensive 

 investigations of the various individual organs. In general, 

 the amount of variation found is sufficient to necessitate great 

 caution in drawing conclusions from a small number of observa- 

 tions (sometimes upon a single animal), as frequently happens 

 in experimental work. 



LENGTHS OF BODY AND TAIL 



The body-length is measured from the tip of the nose to the 

 anus, and the tail-length from the anus to the tip of the tail. 

 The measurements were taken immediately after death, the 

 body and tail being extended by veiy slight tension. Measure- 

 ments during life are not practicable, although they might be 

 obtained by the use of anesthetics. 



In order therefore to discern the changes in the lengths of 

 body and tail while the body-weight is held constant, it was 

 necessary first to determine these measurements on the normal 

 animal. For this purpose, 450 observations (267 males, 183 

 females) were available, varying from newborn to about 400 

 grams body-weight. Of these, 277 (130 males, 147 females) 

 were from the Missouri rats described in a previous paper (.lack- 

 son '13), and 25 (13 males, 12 females) from Mhuiesota. For 



