PRENATAL GROWTH OF SPINAL CORD 41 



Sections of these embryos were magnified by means of an 

 Edinger projection apparatus (E. Leitz-Wetzlar) and outline 

 drawings made of cross-sections of the spinal cords. In the 

 case of the 11 mm., 17 mm., 31 mm., and 65 mm. embryos, every 

 fourth section was drawn, while in the embryo of 150 mm. only 

 every tenth section was used. In exceptional cases where the 

 section to be drawn was torn or distorted, the adjacent section was 

 drawn instead. The magnification used was 50 diameters, which 

 corresponds to a magnification of 2500 times in cross-sectional 

 area. 



The areas of these drawings were measured with a planimeter 

 (Coradi). This instrument on being tested showed an error of 

 less than 0.25 per cent. Two entirely independent readings were 

 made in each case and the average used in order to minimize the 

 error. The percentages of the gray matter (anterior and poste- 

 rior horns), of the white matter (anterior, lateral, and posterior 

 columns), and of the ependyma with the canal were first calculated 

 from the original readings. The original readings were then 

 reduced to their actual size from which again the percentages of 

 the various parts were calculated. This gave a check on the 

 accuracy of the calculation. Since the thickness of the sections 

 was known, it was easy to calculate the total volumes of the cords 

 and their various parts. 



The first section which showed filaments of the first pair of 

 spinal nerves was taken as the upper level of the spinal cord. 

 This, however, was not always the exact upper level, on account 

 of the obliquity of the section caused by the normal curvature of 

 the younger embryos. This error is in most cases not large 

 enough to interfere seriously with the general results, but should 

 be borne in mind. 



The length of a segment was determined by taking all sections 

 between the uppermost point of attachment of a nerve to the 

 cord and the corresponding point of the next pair of nerves caudal 

 to the first. Since the thickness of the sections was known, the 

 lengths and volumes of the various segments were readily calcu- 

 lated. Owing to the curvature of the younger embryos it was 

 impossible in these to obtain the exact length of the upper and the 



