344 SPINAL CORD. 



of the central canal is not very different to what it was at an earlier 

 period. This condition of the spinal cord is especially instructive, 

 as it is very nearly that which is permanent in Amphioxns. 



The next event of importance is the formation of the ventral or 

 anterior fissure. This owes its origin to a downgrowth of the anterior 

 horns of the cord on each side of the middle line. The two down- 

 growths enclose between them a somewhat linear space the anterior 

 fissure which increases in depth in the succeeding stages (fig. 246, a/). 



The dorsal or posterior fissure is formed at a later period 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 me to be still involved 

 in some obscurity. 



In the Elements of Embryology the development of the posterior fissure 

 was described in the following way : 



" On the seventh day the most important event is the formation of the 

 posterior fissure. 



" This is brought about by the absorption of the roof of the posterior of 

 the two parts into which the neural canal has become divided. 



" Between the postei'ior horns of the cord, the epithelium forming the 

 roof of the, so to speak, posterior canal is along the middle line covered 

 neither by grey nor by white matter, and on the seventh day is partially 

 absorbed, thus transforming the canal into a wedge-shaped fissure, whose 

 mouth however is seen in section to be partially closed by a triangular 

 clump of elongated cells (tig. 246, c). Below this mass of cells the fissure 

 is open. It is separated from the ' true spinal canal ' by a very narrow 

 space along which the side walls have coalesced. In the lumbar and sacral 

 regions the two still communicate. 



" We thus find, as was first pointed out b}>- Lockliart Clarke, that the 

 anterior and posterior fissures of the spinal cord are, morphologically speak- 

 ing, entirely different. The anterior fissure is merely the space left between 

 two lateral downward growths of the cord, while the posterior fissure is 

 part of the original neural canal separated from the rest of the cavity 

 (which goes to form the true spinal canal) by a median coalescence of the 

 side walls." 



1 confess that I have some doubts as to the complete accuracy of the 

 above statement. 



Kiilliker gives a full account of the gradual atrophy of the central 

 canal ; but I do not fully understand his statements with reference to the 

 formation of the posterior fissure, which in fact appears to be only inci- 

 dentally mentioned. It would seem from his account that a shallow and 

 somewhat wide dorsal fissure is formed to start with, in the human embryo, 

 by two projections of the postei'ior white horns. On the atrophy of the 

 central canal this furrow becomes narrowed, but Kolliker does not 

 definitely state how it becomes deepened so as to give rise to the permanent 

 dorsal fissure. 



It seems to me 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. 



