IN ELASMOBRANCH FISHES. 17 1 



The upper ends of the muscle-plates have grown no further 

 round the neural canal than in fig. A, and no scattered meso- 

 blastic connective-tissue cells are visible. 



In fig. A the dorsal surface of the neural canal was as com- 

 pletely rounded off as the ventral surface ; but in fig. B III this 

 has ceased to be the case. The cells at the dorsal surface of 

 the neural canal have become rounder and smaller and begun 

 to proliferate, and the uniform outline of the neural canal has 

 here become broken (fig. B III, pr). The peculiar membrane 

 completely surrounding the canal in fig. A now terminates 

 just below the point where the proliferation of cells is taking 

 place. 



The prominence of cells which springs in this way from the 

 top of the neural canal is the commencing rudiment of a pair 

 of spinal nerves. In fig. B II, a section anterior to fig. B III, 

 this formation has advanced much further (fig. Bli,/r). From 

 the extreme top of the neural canal there have now grown out 

 two club-shaped masses of cells, one on each side ; they are 

 perfectly continuous with the cells which form the extreme top 

 of the neural canal, and necessarily also are in contact with 

 each other dorsally. Each grows outwards in contact with the 

 walls of the neural canal ; but, except at the point where they 

 take their origin, they are not continuous with its walls, and are 

 perfectly well separated by a sharp line from them. 



In fig. B I, though the club-shaped processes still retain their 

 attachment to the summit of the neural canal, they have become 

 much longer and more conspicuous. 



Specimens hardened in both chromic acid (PI. 22, fig. C) and 

 picric acid give similar appearances as to the formation of these 

 bodies. 



In those hardened in osmic acid,, though the mutual relations 

 of the masses of cells are very clear, yet it is difficult to dis- 

 tinguish the outlines of the individual cells. 



In the chromic acid specimens (fig. C) the cells of these 

 rudiments appear rounded, and each of them contains a large 

 nucleus. 



I have been unable to prepare longitudinal sections of this 

 stage, either horizontal or vertical, to shew satisfactorily the 

 extreme summit of the spinal .cord ; but I would call attention 



