IN ELASMOBRANCH FISIIKS. 



I have, however, employed the comparison of the Vertebrate 

 and Annelidan nervous cords, not so much to prove a genetic 

 relation between the two as to shew the a priori possibility of 

 the formation of a spinal canal and the a posteriori evidence we 

 have of the Vertebrate spinal canal having been formed in the 

 way indicated. 



I have not made use of what is really the strongest argument 

 for my view, viz. that the embryonic mode of formation of the 

 spinal canal, by a folding in of the external epiblast, is the very 

 method by which I have supposed the spinal canal to have been 

 formed in the ancestors of Vertebrates. 



My object has been to suggest a meaning for the peculiar 

 primitive position of the posterior roots, rather than to attempt 

 to explain in full the origin of the spinal canal. 



EXPLANATION OF THE PLATES 1 . 



PLATE 22. 



Fig. A. Section through the dorsal region of an embryo of Scy Ilium iti I/arc, with 

 the rudiments of two visceral clefts. The section illustrates the general features at a 

 period anterior to the appearance of the posterior nerve-roots. 



nc. neural canal, nip. muscle-plate, c/i. notochord. x. subnotochordal rod. 

 ao. rudiment of dorsal aorta, so. somatopleure. sp. splanchnopleure. al. alimentary 

 tract. All the parts of the section except the spinal cord are drawn somewhat 

 diagrammatically. 



Figs. B i, B n, B in. Three sections of a Prisfiurus-embryo. B I is through 

 the heart, B II through the anterior part of the dorsal region, and B in through 

 a point slightly behind this. Drawn with a camera. (Zeiss CC ocul. 2.) 



In B III there is visible a slight proliferation of cells from the dorsal summit of the 

 neural canal. 



In B II this proliferation definitely constitutes two club-shaped masses of cells (pr), 

 both attached to the dorsal summit of the neural canal. The masses are the rudi- 

 ments of the posterior nerve-roots. 



In B I the rudiments of the posterior roots are of considerable length. 



1 The figures on these Plates give a fair general idea of the appearance presented 

 by the developing spinal nerves ; but the finer details of the original drawings have in 

 several cases become lost in the process of copying. 



The figures which are tinted represent sections of embryos hardened in osmic 

 acid ; those without colour sections of embryos hardened in chromic acid. 



B. I 





