DEVELOPMENT OF ELASMOBRANCH FISHES. 173 



end of the abdominal cavity. The anterior ganglia are the 

 largest ; the commissural cord, if developed, is still very indistinct. 

 In stage L the commissural cord becomes definite, though not 

 very easy to see even in longitudinal sections, and the ganglia 

 become so considerable as not to be easily overlooked. They 

 are represented in PI. XIT. fig. 1, sy. g. and in PI. xvii. fig. 2 

 in the normal j)osition immediately above the cardinal veins. 

 The branches connecting them with the trunks of the spinal 

 nerves may still be seen without difficulty. In later stages these 

 branches cannot so easily be made out in sections, but the 

 ganglia themselves continue as fairly conspicuous objects. The 

 segmental arrangement of the ganglia is shewn in PI. xvii. 

 fig. 3, a longitudinal and vertical section of an embryo between 

 stages L and M with the junctions of the sympathetic ganglia 

 and spinal nerves. The gang-lia occupy the intervals bet^Yeen 

 the successive segments of the kiilneys. 



The sympathetic system anly came under my notice at a 

 comparatively late period in my investigations, and the above 

 facts do not in all points clear up its development \ My obser- 

 vations seem to point to the sympathetic s^^stem arising as an 

 off-shoot from the cerebrospinal system. Intestinal branches 

 would seem to be developed on the main nerve stems of this in 

 the thoracic and abdominal regions, each of these then developes 

 a ganglion, and the ganglia become connected by a longitudinal 

 commissure. On this view a typical spinal nerve has the follow- 

 ing parts : two roots, a dorsal and ventral, the dorsal one 

 ganglionated, and three main branches, (1) a ramus dorsalis, 

 (2) a ramus ventralis, and (3) a ramus intestinalis. This scheme 

 may be advantageously compared with that of a typical cranial 

 nerve according to Gegenbaur. It may be noted that it brings 

 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 wa}'", do not definitely exclude a completely 

 different method of development for the sympathetic system. 



1 The formatiou out of tlie sympathetic gangha of the so-called pau-ecl supra- 

 renal bodies is dealt with iu couuection with the vascular system. The original 

 views of Leydig on these bodies are fully borne out by the facts of their develop- 

 ment. 



