OP ARTS AND SCIENCES. 99 



Iq the median portion of this region it is exceedingly thin, whereas on 

 either side it presents swellings which in cross section have a knob-like 

 appearance. In this region the epithelium is not covered by either the 

 gray or the white substance of the cord. 



In embryos of nine or ten weeks a cross section of the cord shows, 

 according to KoUiker, the posterior columns raised into two parallel 

 ridges. The floor of the shallow furrow embraced between them, which 

 he says is the actual posterior fissure, is formed by the outwardly con- 

 vex surface of the epithelial lining of the central canal. 



Later these elevations come to lie close together, so that only a nar- 

 row slit is found between them ; yet they do not coalesce. Even in 

 the third month there is a connective-tissue partition between them. 

 They become more and more reduced to the level of the surface of the 

 cord, and a kind of separation takes place within, which results in 

 splitting them off as two wedge-shaped bands, which he calls the 

 GoU'sche Keilstrange. 



No account is given of the manner in which the furrow is deepened. 



Clarke ('G2, p. 916) describes the formation of the posterior fissure 

 as follows : — 



" At the commencement of these changes," i. e. those which lead to 

 the formation of the posterior fissure, " the central canal reaches the 

 surface of the posterior gray substance. The growth of this substance 

 is then continued, not only backward, but outward from the mesial line, 

 while in the intervening angular and gradually increasing space be- 

 tween it and this line are developed on each side two new pyramidal 

 columns of longitudinal fibres, which increase in depth in a corre- 

 sponding proportion. 



"Of these the outer one, which is much the larger, rests on the 

 back of the cornu, over which it ultimately blends with the outer por- 

 tion of the posterior columns previously developed. 



" The inner- and smaller column is in general more conspicuous 

 and distinct in the dorsal and cervical than in the lumbar region. 

 The opening between these additional columns constitutes the pos- 

 terior median fissure, which is now occupied by blood and pia 

 mater in connection with radiating fibres from the central epithelial 

 layer." 



Foster and Balfour (74, p. 187) give a very different account of 

 the formation of the fissure. In the embryo chick of seven days the 

 central canal is divided, according to these authors, into two parallel 

 tubes by the median coalescence of its lateral walls, thus forming a 

 dorsal and a ventral canal. Afterwards the roof of the dorsal canal is 



