THK SPINAL NERVES. 27 B 



at a slightly later date, but at a relatively earlier stage in the for- 

 mation of the spinal cord. At a time when the neural canal has 

 hardly commenced to form, and the neural plate is only very 

 slightly folded on itself in the middle line, tire nerve rudiments 

 may be recognised in transverse sections (Fig. 117, MA), as small 

 conical masses of cells, cut out from the deeper part of the epiblast, 

 at the edges of the neural plate. As the neural folds rise up, and 

 grow in towards each other, the nerve rudiments are carried up 

 with the folds to the dorsal surface of the spinal cord, and then 

 complete their development in the manner already described. 



In the posterior half, or so, of the body there appears to be no 

 continuous neural ridge developed, the nerve rudiments being, 

 from the first, independent outgrowths. There are consequently 

 no longitudinal commissures connecting these hinder nerves, 

 similar to those in the anterior part of the body (Fig. 115). 

 These commissures disappear in the anterior part of the body 

 shortly after the fifth day, and it is possible that their absence in 

 the hinder part of the body is to be explained as due to abbrevia- 

 tion of the developmental history, by omission of this stage. 



The ventral roots of the spinal nerves arise later than the 

 dorsal roots, during the latter part of the third day. They appear 

 as small outgrowths from the lower part of the sides of the spinal 

 cord, and from the first occupy the position held by them in the 

 adult. This position is indicated, before the actual appear- 

 ance of the root, by a slight convergence of the cells at the side 

 of the cord ; and the nerve root is apparently formed by the direct 

 outgrowth of processes from these cells, which, passing out from 

 the side of the spinal cord, become the axis cylinders of the nerve 

 fibres. 



Each ventral root arises by a number of separate rootlets, 

 which leave the spinal cord in a longitudinal series, the total 

 length of a root being about equal to half that of a somite ; the 

 root lies opposite the anterior half of the somite, and vertically 

 below the corresponding dorsal root. 



Towards the end of the third day (Fig. 124), the ventral 

 roots, growing downwards and outwards, meet the dorsal roots, 

 and with these form the trunks of the spinal nerves. Beyond 

 the place of union of the roots the nerves continue their growth 

 outwards and downwards, lying along the inner surfaces of 



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