PRENATAL GROWTH OF SPINAL CORD 51 



is a slight increase in the area of cross-section in the cervical 

 region (as compared with the thoracic) which probably corre- 

 sponds to the cervical enlargement. The small increase shown 

 by the lower thoracic and upper lumbar segments is due for the 

 most part to the curvature of the cord. The 31 mm., 65 mm., 

 and 150 mm. embryos all seem to correspond to the general 

 description (tables 4 to 6 and figs. 6 to 12). The 65 mm. embryo 

 shows a larger area of cross-section in the lumbar region than in 

 the cervical. In the others the cervical region is the largest. This 

 is true for the cross-sectional area of the whole cord as well as for 

 the gray matter. 



3. Growth as a whole 



The total volumes of gray matter, both absolute and relative, 

 are given in table 1. The gray matter comprises about 38 per 

 cent of the entire cord in the 11 mm. embryo. In the 17 mm. 

 embryo it has increased in absolute volume approximately 300 

 per cent and now comprises about 50 per cent of the entire cord. 

 The increase in absolute volume continues, but relatively slower, 

 to the 65 mm. embryo, where it forms 58 per cent of the total 

 cord. In the 150 mm, embryo (at the mid-fetal period) the 

 relative volume shows a slight decrease (to 53 per cent), while 

 the absolute volume continues to increase. This relative de- 

 crease continues into postnatal life, until in the child the gray 

 matter forms only 27 per cent of the whole cord and in the adult 

 only 20 per cent. 



As previously stated, the anterior horns are much larger than 

 the posterior in the earlier stages (figs. 1 to 5). The anterior 

 horns can be distinguished in the 11 mm. embryo, although they 

 are far from the characteristic shape assumed later. His ('86) 

 recognized the anlages of the posterior and anterior horns early 

 in the second month, but states that they do not assume their 

 definite form until later, being very broad at three months. 

 Minot ('92) found them fused at about five months. Streeter 

 ('11) and Bryce ('08), however, state that in the 15 mm. embryo 

 (fifth week) the rudiments of the anterior horns can be seen. 



