VIII.J THE POSTERIOE FISSURE. 255 



what linear space — the anterior fissure — which in- 

 creases in depth in the succeeding stages (Fig. 83, a/). 



The dorsal or posterior fissure is formed at a later 

 period (about the seventh day) than the anterior, and 

 accompanies the atrophy of the dorsal section of the 

 embryonically large canal of the spinal cord. The exact 

 mode of its formation appears to be still involved in 

 some obscurity. 



It seems probable, though further investigations on the point 

 are still required, that the dorsal fissure is a direct result of the 

 atrophy of the dorsal part of the central canal of the spinal 

 cord. The walls of this coalesce dorsally, and the coalescence 

 gradually extends inwards, so as finally to reduce the central 

 canal to a minute tube, formed of the ventral part of the original 

 canal. The epithelial wall formed by the coalesced walls 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 grey or 

 white matter, so that, with the gradual reduction of the dorsal 

 part of the canal and the absorption of the epithelial wall formed 

 by the fusion of its two sides, a fissure between the two halves of 

 the spinal cord becomes formed. This fissure is the posterior or 

 dorsal fissure. In the process of its formation the white matter 

 of the dorsal horns becomes prolonged so as to line its walls ; and 

 shortly after its formation the dorsal grey commissure makes its 

 appearance ; this is not improbably derived from part of the 

 epithelium of the original central canal. 



Meanwhile an alteration is taking place in the ex- 

 ternal outline of the cord. From being, as on the 

 fourth and fifth days, oval in section, it becomes, chiefly 

 through the increase of the white matter, much more 

 nearly circular. 



By the end of the seventh day the following im- 



