50 MAX MAYO MILLER 



In comparing the rates of growth in volume and in average 

 cross-sectional area of the spinal cord as a whole, let it be assumed 

 that the volume and the cross-sectional area in the 11 mm. embryo 

 each equals to 1. Then it is noted that in the 17 mm. specimen 

 the volume has increased relatively twice as much as the area 

 of cross-section. In the 31 nam. embryo the increase in length 

 is to the increase in area as 3 is to 2. This relatively greater 

 increase in volume, over the cross-sectional area, is also found 

 in the 65 mm. and 150 mm. embryos, though not so marked in 

 these. This indicates that the growth in length is relatively 

 greater than in area of cross-section, as previously stated. 



B. GRAY MATTER 



1. General form {tables 2 to 6; figs. 1 to' 12) , 



The gray matter, which constitutes the cellular part of the 

 spinal cord, in the older embryos studied shows an increase in 

 cross-sectional area in the regions of the enlargement as compared 

 with that found in the thoracic region. In the 11 mm. embryo, 

 (table 2) no enlargement is found. The anterior horns form 

 more than one-half the gray matter in all specimens studied 

 (table 7). In the youngest embryo the anterior horn anlage is 

 approximately three times as large as the posterior horn, which 

 agrees with the statement of His ('86). Later, however, the 

 posterior horns approach the anterior in size, the mid-fetal stage 

 showing relations similar to the adult. The lateral horn is not 

 well marked except in the older embryos (figs. 1 to 5). In all 

 cases when present it is included in the measurements with the 

 anterior horn. The gray matter shows much variation in shape 

 in different segments. 



2. Special features in the various regions 



In the 11 mm. embryo (table 2 and fig. 6) there seems to be 

 more gray matter (in cross-sectional area) in the cervical than 

 in the thoracic region, but this is at least in part due to the 

 obliquity of the sections. Jn the 17 mm. embryo (table 3) there 



