54 MAX MAYO MILLER 



is found at the caudal end of the conus medullaris where it forms 

 the sinus rhomboidahs (which is so well marked in birds) . After 

 the 17 mm. stage the canal is decidedly narrower in the thoracic 

 region. 



2. Growth 



The cross-sectional areas of the ependyma and canal are shown 

 in figures 1 to 12, and in tables 1 to 6, while the volumes are given 

 in tables 1 and 9 to 11. The ependyma with the canal in the 

 11 mm. embryo form 49 per cent of the volume of the entire cord. 

 In the 17 mm. embryo they have decreased in relative size to 

 24 per cent but show an absolute increase. From this stage they 

 show a decrease both in relative and in actual size until in the 

 mid-fetal period (150 mm.) they form only 0.59 per cent of the 

 whole cord. This decrease corresponds to the closure of the 

 dorsal part of the central canal as described by His ('86). The 

 ependyma and canal are too small to be shown in the curves 

 after the 65 mm. stage. It appears that the ependyma and canal 

 reach their maximum (absolute as well as relative) size during 

 the second month, decreasing steadily thereafter. This agrees 

 with Streeter ('11) who finds them relatively and absolutely 

 largest in the 15 mm. embryo. Minot ('92), however, says the 

 canal remains stationary from the third to fifth months of pre- 

 natal life. 



It seems that from the 17 mm. to the 65 mm. stage the gray 

 and white matter both grow chiefly at the expense of the canal 

 and ependyma; thereafter the white matter continues to increase, 

 while the gray matter decreases in relative volume (percentage 

 of the entire cord). ■ 



SUMMARY 



Some of the more important observations and conclusions con- 

 cerning the growth of the spinal cord, may be summarized as 

 follows : 



1. In the 11 mm. embryo indications of the cervical enlarge- 

 ment appear. In the 31 mm. embryo the lumbar enlargement 

 is first definitely shown, though it may also be present at 17 mm. 



