346 'Journal of Comparative Neurology and Psychology. 



of course, the colony has been recruited from outside sources. 

 By thus extending the observations over a long period the material 

 has lost perhaps a shade in homogeneity, but on the other hand, 

 something has been gained for the general value of the results. 



In collecting the data, I have been assisted by my students and 

 members of the laboratory staff, and I desire on this occasion to 

 acknowledge my indebtedness to those who have worked with me. 

 I am indebted chiefly to my colleague. Dr. Hatai, who has been of 

 the greatest aid in the mathematical treatment of the observations, 

 since without this assistance, the publication of the results must 

 have been delayed still longer. 



In presenting the observations, the effort has been made to 

 condense them as much as possible, while at the same time fur- 

 nishing all the facts '\^hich would enable other observers to control 

 the conclusions. To this end, there is printed a complete table 

 of the individual observations. (See General Table at the end 

 of this paper.) All the formulae and the descriptions of the meth- 

 ods by which the data have been treated are of course given, and 

 in addition, the results have been condensed as usual in the form 

 of curves or tables. The formulas are given only once in each 

 instance, and then referred to by number when they reappear. 



It has not been deemed necessary, however, in view of the 

 general table, to print at the same time correlation tables or the 

 intermediate calculations. 



TECHNIQUE. 



It is to be expected that during so long a period the methods 

 of observation should have changed somewhat and also should 

 have been improved. In giving the technique for removing the 

 brain and spinal cord and for making the other measurements 

 the methods described are those now used, it being understood 

 that if, in any instance, there was previously a deviation from the 

 procedure which might modify the results, this fact has been 

 taken into account. 



The procedure was as follows: Just before feeding time, i.e., 

 when the stomach is comparatively empty, the rat was chloro- 

 formed and notes made on the age, sex and any important con- 

 ditions which might have modified the development of the animal. 

 It was then weighed to the tenth of a gram, and the body length 



