340 0^^' \yoi.. I. 



to be simply attenuated polar outgrowths connecting the gan- 

 glion-cells of one lateral wall with the ganglion-cells of the other. 

 In longitudinal section it may be seen that these fibres crossing 

 the ventral surface of the tube run parallel with each other, and 

 exactly at right angles to the long axis of the tube. I have not 

 been able to find any trace of their bending horizontally to join 

 the course of the longitudinal fibres. Whatever connection may 

 take place between the transverse and longitudinal fibres, seems 

 to occur only through the mediation of ganglion-cells. The 

 ventral transverse commissure is continuous through the body- 

 region, the hind-brain and mid-brain, as far forward as the ven- 

 tral angle caused by the cranial flexure just in front of the exit 

 of the third nerve-pair. Anterior to this angle is the infundibu- 

 lar region, where no nerve-fibres appear. The ventral commis- 

 sure, in its upward lateral course, cuts off from the main mass a 

 small bundle of ganglion-cells, which are marked ^c in Fig. ^"j. 

 From this continuous bundle of ganglion-cells spring the fibres 

 which enter the ventral nerve-roots. The ventral lateral longi- 

 tudinal band of fibres as it grows in thickness gradually extends 

 upward until it meets and blends with the dorsal band, leaving 

 only the small spaces where the fibres of the dorsal nerve-roots 

 pass out from the main mass of ganglion-cells. Neither the 

 longitudinal nor the transverse fibres enter the nerve-roots. In 

 the cranial as well as in the spinal region the fibres which enter 

 the nerve-roots pass from the neighboring ganglion-cells through 

 the lateral band to the roots. 



Fig. 6"] , PI. XVI., represents a stage in the development of 

 the spinal cord later than that of Fig. 66. In Fig. 6^ it will be 

 seen that the lateral bands have coalesced, and their continued 

 growth causes on each side a ventral protuberance (//). These 

 protuberances gradually extend in a ventral median direction, 

 and form the lateral walls of the ventral fissure. The ventral 

 commissure remains the roof of the ventral (anterior) fissure. 

 In this way the ventral commissure comes to lie eventually just 

 below the middle of the cord. The dorsal fissure arises by the 

 median compression of the lateral walls of the central canal, 

 and the atrophy of the roof of the latter, so that at a slightly 

 later stage there may be traced a straight line dividing the lat- 

 eral halves of the cord, and running from the dorsal surface 

 down to the remnant of the central canal. The remnant of the 



