56 MAX MAYO MILLER 



sacral region from the 31 mm. stage, is slightly greater than that 

 in the thoracic region. The anterior horn is about three times 

 as large as the posterior horn in the youngest embryo (11 mm.). 

 This difference in size becomes less 'in the later stages where the 

 ratio approaches that found in the adult cord. 



5. The white matter has a rate of growth different from that 

 of the gray matter. It increases steadily from 13 per cent in 

 the 11 mm. stage to 46 per cent of the whole cord in the mid- 

 fetal period (150 mm.). In the child it forms 73 per cent and in 

 the adult 80 per cent, showing that the steady increase continues 

 into postnatal life. In the white matter, as also in the gray 

 matter, the relative increase in the different regions is about the 

 same as the increase in the cord as a whole. The different col- 

 umns of white matter present irregularities in growth which may 

 be due to the successive formation of various tracts at different 

 ages. The lateral column appears always the largest, however, 

 especially in the earlier stages. 



6. The ependyma with the canal show some interesting growth 

 relations. In the 11 mm. embryo they form nearly 50 per cent 

 of the entire cord. This is followed by a rapid relative decrease 

 until by middle of the fetal period (150 mm.) they form only 

 0.59 per cent of the whole. This marked relative decrease is 

 accompanied by a decrease in the absolute size from the 17 muL 

 stage onward. With the exception of a slight dilation at the 

 extremities, the canal is fairly uniform in caliber in the 11 mm. 

 and 17 mm. stages, but from the 31 mm. stage onward it is more 

 constricted in the thoracic region. The white and gray matter 

 both grow at the expense of the ependyma and canal until about 

 the third month, when the gray matter begins to decrease in 

 relative amount while the white matter continues to increase. 



