156 'Journal of Comparative Neurology and Psychology. 



differences in this relation according to sex, as well as to make com- 

 parisons with other forms. 



It also permits the determination of the influence of dwarfing and 

 other modifying conditions on the weight-length relation. 



f3. Both the weight of the brain and of the spinal cord can be 

 related to the body length, and the measurement on body length thus 

 made to furjiish an additional datum from which the weights of the 

 brain and of the spinal cord can be inferred. As we shall see, this 

 datum is a much better one than body weight, especially in those 

 cases where, for one reason or another, the animal has become 

 emaciated. 



4. If we consider the body length of the rat to correspond in a 

 general way with the sitting height in man, we have one more 

 means of comparing the growth changes in the two forms. 



In the following pages we shall discuss these points, so far as 

 they have been worked out. For the mathematical treatment of 

 the results I am indebted to my colleague, Dr. Hatai, who is pub- 

 lishing at this same time some notes on the formulas previously 

 used by both of us (Hatai, '08; Donaldson, '08), as well as giving 

 a new and more general formula for determining the weight of the 

 brain from the body weight (Hatai, '09). 



The technique of weighing and measuring was that described in 

 the earlier paper (Donaldson, '08). A number of complete rec- 

 ords on the albino rat have been added to those on hand at that 

 time. Moreover, for the relation of body weight to the body length 

 alone, additional records have been obtained by weighing and meas- 

 uring animals which had been anesthetized lightly. 



It was my first intention to print the full series of individual 

 records (233 males, 173 females) in a general table at the end of 

 this paper. I have, however, decided not to do so for the following 

 reasons : — 



First. — Printing such a general table would involve repeating a 

 number of the records already published in a former paper (Don- 

 aldson, '08), and would in turn need to be again repeated in a forth- 

 coming paper on the change in the percentage of water during the 

 growth of the nervous svstem. 



