GROWTH OF THE CEREBRAL CORTEX 523 



Two or more measurements of each locality on a given section 

 were always made and, where these differed, the mean value was 

 recorded. The same locality measured on the different sections 

 from one and the same series, taken from a given slice, showed 

 for the most part some differences in thickness, though never 

 large differences. The main reasons for these differences appear 

 to be the following: 



1. As 6 to 12 sections were taken in succession from a given 

 sUce, at the level of the mark of india ink, these sections should 

 not be identical as to thickness of the cortex, owing to the fact 

 that the surface of the hemisphere is, of course, not strictly ver- 

 tical to the plane of the section. Changes in cortical thickness 

 due to this cause are, in general, very slight. 



2. Although the outer boundary of the cortex is very distinct, 

 the borderline between the cortex and the white substance is 

 not equally sharp and this may prove a source of slight error in 

 determining the thickness of the cortex. Direct tests indicate a 

 variability of less than one-half of 1 per cent plus or minus, so 

 that this error may be neglected. 



3. In the procedure of attaching sections to slide by means of 

 albumin-glycerine, the sections were first warmed over a small 

 flame, in order to unfold and flatten them. During this manipu- 

 lation, slight differences of extension sometimes occur in conse- 

 quence of the dissimilarity of the temperature used. If the tem- 

 perature be a little higher than the melting point of the paraf- 

 fine, sections extend at first much, and, when they are cooled, they 

 may shrink rather suddenly, thereby reducing the size of sec- 

 tions below that found before heating. If the temperature be 

 raised only just to the melting point, or a little below it, the size 

 of the sections on slide will be quite unmodified, but it is not 

 always easy to control the temperature precisely, and significant 

 alterations in the size of the sections due to this influence do 

 occur. With a view to adjusting these differences, a method of 

 correcting the direct observations was devised and the manner 

 in which the correction was applied will be given in a later 

 chapter. 



