42 MAX MAYO MILLER 



lower segments. This, however, did not interfere with obtaining 

 accurately the total volume of the cord and of the various regions. 

 In the 65 mm. and 150 mm. embryos the uppermost segments 

 were missing. These were estimated by calculation from the 

 other segments of the same cords, assuming that the same relative 

 increase takes place in these segments as in the other segments of 

 the same cord, when compared with the cord of the 31 mm. em- 

 bryo. While the obliquity of some of the sections interfered in 

 some respects, these exceptions are carefully noted so that they 

 have not resulted in any great error in the accuracy of the work. 

 The curvature and corresponding obliquity of cross-sections can 

 be determined approximately by the graphic reconstruction in 

 lateral view of the four younger embryos in the paper by Jackson 

 ('09 a). 



The exact line of demarcation of gray from white matter was 

 sometimes difficult to determine, due to their intermingling. 

 In the younger embryos in which only the anlages of the anterior 

 and posterior horns are present, a horizontal line was drawn from 

 the small recess in the boundary zone of the gray matter, which 

 was very thin, to the nearest point of the central canal. This 

 line arbitrarily separated the anterior from the posterior horns. 

 The lateral horn was not present in the younger embryos. In the 

 older stages the lateral horn was included with the anterior, thus 

 dividing the gray matter into a posterior and an antero-lateral 

 horn (figs. 1 to 5). 



The white matter was separated into the anterior, posterior, 

 and lateral columns. The lateral border of the anterior column 

 is the line of emergence of the outermost fascicles of the nerve 

 roots. This separates the anterior and lateral columns. The 

 dorso-lateral sulcus at the attachment of the posterior nerve roots 

 separates the posterior and lateral columns. In the 11 mm. 

 specimen the lateral columns showed such an irregularity that 

 they were not separated but measured with the anterior columns. 

 For exact lines of demarcation, see figures 1 to 5. 



