106 PROCEEDINGS OF THE AMERICAN ACADExMY 



The posterior fissure, so called, is carried ventrally with the Bur- 

 dach'sche Keilstrilnge, either contemporaneously with this restriction of 

 the canal, though more slowly, or else not until after the restriction is 

 far advanced. 



REDUCTION OF THE CENTRAL CANAL. 



Literature. 



According to most authors the central canal of the spinal cord in 

 the adult is formed from the ventral portion of the primitive canal. 

 Thus Kcilliker ('Gl, p. 261) says that the primitive canal undergoes a 

 gradual atiophy from the dorsal edge ventralwards, and that its oblit- 

 eration is due to the increase in size of the ))osterior columns. 



Balfour and Foster ('74, p. 186) claim that in the chick the canal 

 is divided into two portions, a dorsal and a ventral, by the median co- 

 alescence of its walls, and that the ventral becomes the permanent 

 canal. 



Waldeyer ('76, p. 23) takes the same view as Kolliker. 



The latter still ('79, p. 590) maintains the same opinion as in his 

 previous writings on this topic. 



According to Loewe ('80, p. 1 1 9), however, the primitive canal suf- 

 fers a reduction of its caliber both dorsally and ventrally. He bases 

 his conclusion on the fact that the epithelial cells at the anterior (ven- 

 tral) edge of the canal have an arrangement of nuclei and fibres simi- 

 lar to that of the cells at the posterior edge ; i. e. the nuclei are situated 

 at the peripheral ends of the cells, while the central ends have a clear 

 fibrous appearance. He says that the fibrous ends of these epithelial 

 cells coalesce just as they do on the dorsal side, and that they are in a 

 similar way transformed into horn fibres. 



Balfour ('81, p. 345) says that the walls of the canal coalesce dor- 

 sally, and that the coalescence then proceeds ventralwards, so that 

 finally it reduces the canal to a minute tube formed of the ventral part 

 of the original canal. 



Observations. 



It can be shown from a series of measurements of the ab.^olute 

 length of the epithelium forming the floor of the canal, together with 

 measurements of the di.«tance from the lumen of the canal to the bot- 

 tom of the anterior fissure, that there is neither a gradual change in the 

 former nor a corresponding increase of the latter, as would naturally 

 be expected if the canal closes along the ventral as well as the dorsal 



