100 PROCEEDINGS OP THE AMERICAN ACADEMY 



partially absorbed, thus converting the canal into a wedge-shaped fissure, 

 whose mouth, however, is closed by a triangular clump of elongated 

 cells. Below this mass of cells the fissure is open. In the lumbar and 

 sacral regions the two canals still communicate. 



Thus the posterior fissure is claimed to be part of the lumen of the 

 orisxinal neural canal. 



Lubinoff' ('74, PI. VII.) figures the same elevations of the posterior 

 columns that Kcilliker has described, but does not deal with the devel- 

 opment of the fissure. 



Waldeyer (76, p. 23) says that the canal closes dorsally by an 

 ingrowth of cells. Afterwards the median jiart of the posterior col- 

 umns grows towards the central canal on each side of the median 

 plane. These always grow from the already formed parts of the 

 posterior columns. 



After reproducing the substance of his earlier account of the for- 

 mation of the fissure in man, KoUiker (79, p. 597) says that in the 

 embryo rabbit the posterior columns, after reaching, on the seven- 

 teenth day, the posterior median line, grow inwards toward the central 

 canal, apparently without the formation of any Keilstriinge. 



In the case of the sheep, Loewe ('80, p. 75) finds the first trace of 

 the posterior fissure in embryos of 1.5 cm. The Goll'sche Keilstriinge 

 also appear at this stage. Between the pyramids there are fibres cor- 

 responding to the anterior horn fibres. They later develop into the 

 posterior horn fibres. 



In embryos of 2 cm. the Keilstriinge occur as two lateral forma- 

 tions, rounded ventrally, and lying close to one another. They are 

 somewhat divergent behind, thus embracing an opening, the posterior 

 median fissure. The Keilstriinge are formed very late, but develop 

 fast in comparison with the other columns. He does not state in 

 what manner this fissure becomes deepened. 



Balfour ('81, p. 344) confesses that he has "some doubts as to the 

 complete accuracy " of the conclusions to which Foster and he had 

 previously arrived ('74, p. 187). He thinks it probable that the dor- 

 sal fissure is a direct result of the atrophy of the dorsal part of the 

 central canal. Dorsally the walls of the canal coalesce, and the fusion 

 gradually proceeds ventralwards, so as to reduce the canal to a minute 

 tube. The epithelial wall formed by this fusion on the dorsal side of 

 the canal is gradually absorbed. 



" The epithelium of the central canal at the period when its atrophy 

 commences is not covered dorsally either by gray or white matter, so 

 that with the gradual reduction of the dorsal part of the canal and the 



