394 DEVELOPMENT OF ELASMOBRANCH FISHES. 



It appears to me, (if the difficulties of comparing the 

 Annelidan ventral cord with the spinal cord of Vertebrates are 

 found to be insurmountable), that this hypothesis would involve 

 far fewer improbabilities than one which supposes the whole 

 central nervous system of Vertebrates to be homologous with 

 the super-cesophageal ganglia. The mode of formation of a 

 nervous system presupposed in my hypothesis, well accords with 

 what we know of the formation of the ventral cord in existing 

 Annelids. 



The supposition of the existence of another branch of seg- 

 mented Vermes is not a very great difficulty. Even at the 

 present day we have possibly more than one branch of Vermes 

 which have independently acquired segmentation, viz.: the 

 Chaetopodous Annelids and the Hirudinea. If the latter is an 

 isolated branch, it is especially interesting from having inde- 

 pendently developed a series of segmental organs like those of 

 Chaetopodous Annelids, which we must suppose the ancestors of 

 Vertebrates also to have done if they too form an independent 

 branch. 



In addition to the difficulty of imagining a fresh line of 

 segmented Vermes, there is another difficulty to my view, viz. : 

 the fact that in almost all Vermes, the blood flows forwards 

 in the dorsal vessel, and backwards in the ventral vessel. This 

 condition of the circulation very well suits the view of a change 

 of the dorsal for the ventral surfaces, but is opposed to these 

 surfaces being the same for Vertebrates and Vermes. I cannot 

 however regard this point as a very serious difficulty to my view, 

 considering how undefined is the circulation in the unsegmented 

 groups of the Vermes. 



Sympathetic nervous system. 



Between stages K and L there may be seen short branches 

 from the spinal nerves, which take a course towards the median 

 line of the body, and terminate in small irregular cellular masses 

 immediately dorsal to the cardinal veins (PI. 18, fig. I, sy. g.\ 

 These form the first traces that have come under my notice of 

 the sympathetic nervous system. In the youngest of my embryos 

 in which I have detected these it has not been possible for me 



