GKOWTH OF THE CEREBRAL CORTEX 513 



II. MATERIAL 



The animals used in the present study were all from the rat 

 colony of The Wistar Institute. I employed 124 albino rats, 96 

 males and 28 females, representing every stage of postnatal 

 growth and having approximately standard body weights. Af- 

 ter .dissection, the entire brain was separated from the spinal 

 cord by a section at the level of the calamus scriptorius and was 

 weighed in a small weighing bottle. I have classified the indi- 

 viduals in twenty groups, according to the number of decigrams 

 in the brain weight of each, for example, brains weighing 0.300 

 to 0.399 grams form Group III and brains weighing 1.500 to 

 1.599 grams. Group XV, etc. In each group, the individual was 

 designated as a, b, c in the order of the date of the dissection. 

 The body and brain weights, body and tail lengths, sex and age 

 of each individual are given in tables 1 and 2. The measure- 

 ments were made by the usual methods employed at The Wistar 

 Institute. 



My reason for classifying the material according to brain weight, 

 is that my own observations as well as those of others have shown 

 that the brain weight increases most regularly according to age, 

 and is much more resistant to outside influences than the body 

 weight, for example. Hence it seemed to me to be not only more 

 cojivenient, but also more precise to arrange the material ac- 

 cording to the brain weight, rather than according to the body 

 weight, or some other physical character. 



Tables 1 and 2 show the sex, body and tail lengths, body and 

 brain weights of the albino rats used in this study on the thick- 

 ness of the cerebral cortex (table 1) in the sagittal and the frontal 

 sections and (table 2) in the horizontal sections respectively, 

 entered in the order of the increasing brain weight. 



These animals were collected at random from many different 

 litters and at various seasons. iVlthough individual variations 

 appear when the data are compared with the values in the ref- 

 erence tables for the rat (Donaldson, '15), nevertheless, by group- 

 ing these data according to the number of decigrams of brain 

 weight, and obtaining the average values, it is found that these 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 28, NO. 3 



