OP ARTS AND SCIENCES. 107 



side. There is, it is true, a variation in the thickness of the epithelium 

 and in the other measurements, but it shows no gradation correlated to 

 the gradual increase in the size of the cord. (Compare table, p. 108.) 



Moreover, the epithelium in this region shows no marked histologi- 

 cal change accompanying the growth of the cord. None of the changes 

 which accompany the atrophy of the canal in its dorsal portion are vis- 

 ible here. There is neither a thickening of the cuticula interna on the 

 ventral side of the canal corresponding to that which is constant along 

 the apex of the dorsal side, nor is there a partition of horn fibres mark- 

 ing the line of fusion of the side walls of the canal, such as exists on 

 the dorsal side of the canal . 



In general, the canal has a broadly rounded floor, which contrasts 

 strongly with the often fissure-like condition of the roof. This we 

 should not expect to find if the canal closed from its ventral as well as 

 from its dorsal side. 



Rf:SUMfi. 



The posterior columns after covering the posterior segment of the 

 cord develop inwards towards the central canal as two horns known as 

 Burdach's Keilstrange. They do not abut upon one another at first, 

 but embrace between their approximated edges two masses of cells, one 

 on either side of the median plane, which later become metamorphosed 

 into Goll's Keilstriinge. The latter are therefore developed indepen- 

 dently of the posterior columns, and are not split off from them as Kol- 

 liker ('Gl, p. 262) has maintained. As the posterior horns develop 

 towards the central canal, they carry between them the posterior fissure, 

 which is filled with the horn fibres derived from the epithelial cells of 

 the central canal. *' 



There is neither a median coalescence of the side walls of the canal, 

 as Balfour and Foster (74, p. 186) describe in the chick, nor is the 

 reduction of the canal brought about by the growth of the posterior 

 columns, as Kolliker states ('61, p. 261) ; but the diminution in the 

 caliber of the canal and the development of Burdach's Keilstrange — 

 the latter process being accompanied by the development of the poste- 

 rior fissure — are entirely independent phenomena. 



The nuclei of the white substance originate independently of the 

 blood-vessels. They appear first in those portions of tlie white sub- 

 stance lying nearest the gray matter, and are undoubtedly derived from 

 the latter. 



There is no atrophy of the ventral part of the primitive canal, as 

 Loewe ('80, p. 119) has maintained, but the central canal of the adult 

 represents the ventral portion of the primitive canal. 



