102 PROCEEDINGS OF THE AMERICAN ACADEMY 



The cuticula interna (PI. III. fig. 1, i) presents a marked thickeu- 

 iiiiT alone: the dorsal edge of the still slit-like central canal- 



There is in the dorsal wall of the canal in the median plane a tract 

 which is destitute of nuclei ; it is composed of fibres differentiated from 

 the epithelial cells the nuclei of which lie at their peripheral ends ; the 

 central ends of the fibres abut upon the cuticula interna. These nuclei 

 are granular and oval, the longer axis having a dorso-ventral direction. 

 It seems probable from the arrangement of the fibres and nuclei that 

 the latter remain practically fixed, while the cells to which they be- 

 long — inasmuch as they are attached to the cuticula interna, which 

 retreats with the restriction of the canal — are drawn out into fibres, 

 which later are metamorphosed into the posterior horn fibres. More 

 and more of the cells from the lateral epithelial walls of ihe canal 

 are drawn in towards the median plane as the closure proceeds. 



The posterior white columns (PI. III. fig. 1, h) have not yet reached 

 the dorsal median line, but as seen in cross sections appear to gradu- 

 ally thin out as they approach it. 



In the dorsal region the nuclei are somewhat more elongated, but 

 their extremities are still rounded. The lumen of the canal is of 

 about the same shape as in the cervical region, but it is somewhat less 

 restricted (PI. I. fig. 2). 



In embryos 54 mm. in length cross sections through the cervical 

 region (PI. III. fig. 2) show that the white columns have approached 

 much nearer the median plane ; there is, however, between their prox- 

 imal edges and this plane a space which is filled with a reticulum that 

 is directly continuous with the gray substance. In the deej)er parts 

 of this tract there are many nuclei, but as one approaches the surface 

 of the cord they gradually disappear. The white columns have already 

 begun to grow towards the central canal, there forming two inwardly 

 projecting horns, afterwards known as the Burdach'sche Keilstriinge. 

 In the median plane there is a kind of partition formed of fibres which 

 are derived from the epithelial cells of the primitive canal ; it extends 

 from the dorsal edge of the canal to the surface of the cord. 



PI. III. fiff. 3, shows a section throusrh the lumbar rcsion of the same 

 embryo. Here the posterior columns have not yet begun to develop 

 towards the central canal. The restriction of the canal has not pro- 

 ceeded so far as in the cervical and dorsal regions. Compare PI. I. 

 figs. 7 and 8. 



In the sacral renion (PI. I. fig. 8) the epithelium of the central 

 canal still extends to the surface of the cord. 



The degree to which the posterior columns approximate each other 



