396 DEVELOPMENT OF ELASMOBRANCII FISHES. 



the sympathetic nervous system into accord with the other 

 parts of the nervous system as a product of the epiblast, and 

 derived from outgrowths from the neural axis. It is clear, how- 

 ever, that my investigations, though they may naturally be 

 interpreted in this way, do not definitely exclude a completely 

 different method of development for the sympathetic system. 



EXPLANATION OF PLATE 14. 



This Plate illustrates the Formation of the Spinal Nerves. 



COMPLETE LIST OF REFERENCE LETTERS. 



a r. Anterior root of a spinal nerve, ch. Notochord. com. Commissure connect- 

 ing the posterior roots of the spinal nerves, i. Mesoblastic investment of spinal cord. 

 m p. Muscle-plate. n. Spinal nerve, nc. Neural canal, pr. Posterior root of a 

 spinal nerve, spg. Ganglion on posterior root of spinal nerve, v r. Vertebral rudi- 

 ment, w. White matter of spinal cord. y. Point where the spinal cord became 

 segmented off from the superjacent epiblast. 



Figs, i , 2, and 3. Three sections of a Pristiurus embryo belonging to stage 1. 

 Fig. i passes through the heart, fig. 2 through the anterior part of the dorsal region, 

 fig. 3 through a point slightly behind this. (Zeiss CC, ocul. 2.) In fig. 3 there is 

 visible a slight proliferation of cells from the dorsal summit of the neural canal. In 

 fig. 2 this proliferation definitely constitutes two club-shaped masses of cells (pr) the 

 rudiments of the posterior nerve-roots, both attached to the dorsal summit of the 

 spinal cord. In fig. r the rudiments of the posterior roots are of considerable length. 



Fig. 4. Section through the dorsal region of a Torpedo embryo slightly older 

 than stage I, with three visceral clefts. (Zeiss CC, ocul. 2.) The section shews the 

 formation of a pair of dorsal nerve-rudiments (pr) and a ventral nerve-rudiment (a r). 

 The latter is shewn in its youngest condition, and is not distinctly cellular. 



Fig- 5- Section through the dorsal region of a Torpedo embryo slightly younger 

 than stage K. (Zeiss CC, ocul. 2.) The connective-tissue cells are omitted. The 

 rudiment of the ganglion (spg) on the posterior root has appeared, and the junction of 

 posterior root with the cord is difficult to detect. The anterior root forms an elonga- 

 ted cellular structure. 



Fig. 6. Section through the dorsal region of a Pristiurus embryo of stage K. 

 (Zeiss CC, ocul. 2.) The section especially illustrates the attachment of the posterior 

 root to the spinal cord. 



Fig. 7. Section through the same embryo as fig. 6. (Zeiss CC, ocul. r.) The 

 section contains an anterior root, which takes its origin at a point opposite the interval 

 between two posterior roots. 



Fig. 8. A series of posterior roots with their central ends united by a dorsal 

 commissure, from a longitudinal and vertical section of a Scyllium embryo belonging 

 to a stage intermediate between L and M. The embryo was hardened in a mixture 

 of osmic and chromic acids. 



Fig. 9. The central end of a posterior nerve-root from the same embryo, with the 

 commissure springing out from it on either side. 



