THE SPINAL NERVES. 383 



These are the rudiments of the posterior nerve-roots. The 

 outgrowths, though at first attached to the spinal cord through- 

 out their whole length, soon cease to be so, and remain in 

 connection with it at certain points only, which form the 

 primitive junctions of the posterior roots with the spinal cord. 

 The original outgrowth on each side remains as a bridge, 

 uniting together the dorsal extremities of all the posterior roots. 

 The posterior roots, though primitively attached to the dorsal 

 summit of the spinal cord, eventually come to arise from its 

 sides. The original homogeneous rudiments before the close of 

 stage K become differentiated into a root, a ganglion, and a 

 nerve. 



The anterior roots, like the posterior, are outgrowths from 

 the spinal cord, but are united independently with it, and the 

 points from which they spring originally, remain as those by 

 which they are permanently attached. The anterior roots arise, 

 not vertically below r , but in the intervals between the posterior 

 roots. They are at first quite separate from the posterior roots ; 

 but before the close of stage K a junction is effected between 

 each posterior root and the corresponding anterior root. The 

 anterior root joins the posterior at some little. distance below its 

 ganglion. 



The results here arrived at are nearly in direct opposition to 

 those of the majority of investigators, though in accordance, at 

 least so far as the posterior roots are concerned, with the 

 beautiful observations of Hensen 'on the Development of Mam- 

 malia 1 .' 



Mr Marshall 2 has more recently published a paper on the 

 development of the nerves in Birds, in which he shews in a 

 most striking manner that the observations recorded here for 

 Elasmobranchs hold good for the posterior roots of Birds. The 

 similarity between his figures and my own is very noticeable. 

 A further discussion of the literature would be quite unprofit- 

 able, and I proceed at once to certain considerations suggested 

 by the above observations. 



1 Zeit. f. Anat. u. Entwickhmgsgeschichte, Vol. I. 



- Journal of Anatomy and Physiology, Vol. xi. April, 1877. 



