PRENATAL GROWTH OF SPINAL CORD 53 



up to the 65 mm. embryo but thereafter decreases relatively 

 into the postnatal life. 



The rate of growth for the various columns is somewhat irregu- 

 lar. This may be due to the formation of the different nerve 

 tracts at different periods. Judging from the average cross- 

 sectional areas in table 8, the lateral column in the earlier stages 

 forms more than half of the white matter of the entire cord. Later 

 it decreases in relative size, but is always the largest column, 

 which holds true even in the adult cord. In general the anterior 

 and posterior columns appear relatively small at first, but in- 

 crease later, approaching the adult proportions in the later fetal 

 days. 



3. Growth in the different regions (figs. 1 to 12 and tables 8 to 12) 



The white matter in the cervical region of the 11 mm. and 

 17 mm. embryos, is relatively larger than in the cord as a whole. 

 In the 31 mm. embryo it is about equal and in the 65 mm. and 

 150 mm. specimens it is less. Over 50 per cent of the total white 

 matter of the 11 mm. embryo is in the cervical region. In the 

 older embryos (from 31 mm.) a greater amount of white matter 

 is found in the thoracic region than in any other. The length 

 of the thoracic region accounts for the larger volume, since the 

 area of cross-section in this region is smaller than in either the 

 cervical or lumbar region. In each of the various regions there is 

 a relative increase of white matter present from the youngest 

 embryo to the adult, as found in the entire cord. 



D. EPENDYMA WITH THE CENTRAL CANAL 



1. Form (figs. 1 to 12) 



The ependyma and central canal are measured together in this 

 study. They undergo some very marked changes during pre- 

 natal life. Only the volume and the area in cross-section are 

 considered here. At the cephalic end where it is continuous 

 with the fourth ventricle the canal is usually slightly enlarged. 

 A corresponding enlargement, though somewhat more marked, 



