GROWTH OF THE CEREBRAL CORTEX 497 



from the measurements of which its general shape and size can 

 be closely determined. 



The materials used in this study were all employed for the 

 further investigation on the cortical development, including a 

 study on the thickness of the cerebral cortex. The present re- 

 search was begun in October, 1915, and concluded in June, 1916, 

 at The Wistar Institute of Anatomy and Biology. 



In connection with this study, which is the first of a series 

 made during my stay in Philadelphia, I desire to express my sin- 

 cere thanks to Dr. M. J. Greenman, Director of The Wistar 

 Institute, for extending to me the privileges of the Institute and 

 putting its facilities at my disposal — and also to acknowledge 

 my obligations to Prof. Henry H. Donaldson under whose direc- 

 tion these researches have been made. 



II. MATERIAL AND TECHNIQUE 



The material, consisting of 141 albino rats (106 males, 32 

 females and 3, sex undetermined) — representing every phase of 

 postnatal growth and having approximately standard body 

 measurements — were all from the rat colony at The Wistar In- 

 stitute. After dissection the entire brain was severed from the 

 spinal cord by a transverse section at the level of the calamus 

 scriptorius and weighed to a milligram in a weighing bottle. 

 The brain was next put on a glass plate, base down, without any 

 lateral support. Five diameters of the hemispheres were then 

 measured with a sliding calipers to a twentieth of millimeter, 

 according to the method to be later described. 



These measured values are, of course, slightly different from 

 the values of the same diameters taken on the hemispheres in 

 situ within the skull, because normally the basal surface of the 

 skull does not lie in a plane, but is slightly arched, and the sides 

 are supported by the temporal walls of the skull. Thus, as meas- 

 ured, the height of the fresh brain is somewhat reduced and the 

 width increased. To measure the brain in situ was however 

 not satisfactory and I thought it better to bring the brain to a 

 position convenient for measurement, in order to get more exact 



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



