IN ELASMOBRANCH FISHES. 19 1 



The presence of longitudinal commissures connecting the 

 central ends of all the posterior roots is very peculiar. The 

 commissures may possibly be looked on as outlying portions 

 of the cord, rather than as parts of the nerves. 



I have not up to this time followed their history beyond a 

 somewhat early period in embryonic life, and am therefore un- 

 acquainted with their fate in the adult. 



As far as I am aware, no trace of similar structures has been 

 met with in other vertebrates. 



The commissures have a very strong resemblance to those 

 by which in Elasmobranch Fishes the glossopharyngeal nerve 

 and the branches of the pneumogastric are united in an early 

 embryonic stage 1 . 



I think it not impossible that the commissures in the two 

 cases represent the same structures. If this is the case, it would 

 seem that the junction of a number of nerves to form the pneu- 

 mogastric is not a secondary state, but the remnant of a primary 

 one, in which all the spinal nerves were united, as they embryo- 

 nically are in Elasmobranchs. 



One point brought out in my investigations appears to me 

 to have bearings upon the origin of the central canal of the 

 Vertebrate nervous system, and in consequence upon the origin 

 of the Vertebrate group itself. 



The point I allude to is the posterior nerve-rudiments 

 making their first appearance at the extreme dorsal summit of 

 the spinal cord. 



The transverse section of the ventral nervous cord of an ordi- 

 nary segmented worm consists of two symmetrical halves placed 

 side by side. 



If by a mechanical folding the two lateral halves of the 

 nervous cord became bent towards each other, while into the 

 groove formed between the two the external skin became pushed, 

 we should have an approximation to the Vertebrate spinal cord. 

 Such a folding might take place to give extra rigidity to the 

 body in the absence of a vertebral column. 



If this folding were then completed in such a way that 

 the groove, lined by external skin and situated between the 



1 Balfour, "A Preliminary Account of the Development of Elasmobranch Fishes," 

 Q. y. Micros. Sc. 1874, plate xv. fig. 14, r.g. [This edition, PI. 4, fig. 14, v.g.}. 



