IN ELASMOBRANCH FISHES. 175 



the last stage (PL 22, fig. C, vr) become split off from the 

 inner layer of the muscle-plates. The vertebral rudiments have 

 in fact commenced to grow up on each side of the neural canal, 

 in order to form the mass of cells out of which the neural arches 

 are subsequently developed. 



The dorsal extremities of the muscle-plates form the second 

 source of these connective-tissue cells. These latter cells lie 

 dorsal and external to the nerve-rudiments. 



The presence of this connective tissue, in addition to the 

 nerve-rudiments, removes the possibility of erroneous interpre- 

 tations in the previous stages of the Pristittrus-embryo. 



It might be urged that the two masses which I have called 

 nerve-rudiments are nothing else than mesoblastic connective 

 tissue commencing to develope around the neural canal, and 

 that the appearance of attachment to the neural canal which 

 they present is due to bad preparation or imperfect observation. 

 The sections of both this and the last Torpedo-embryo which 

 I have been describing clearly prove that this is not the case. 

 We have, in fact, in the same sections the developing connective 

 tissue as well as the nerve-rudiments, and at a time when the 

 latter still retains its primitive attachment to the neural canal. 

 The anterior root (fig. D b, ar) is still a distinct conical promi- 

 nence, but somewhat larger than in the previously described 

 embryo ; it is composed of several cells, and the cells of the 

 spinal cord in its neighbourhood converge towards its point 

 of origin. 



In a Torpedo-embryo (PL 22, fig. D c) somewhat older 

 than the one last described, though again derived from the 

 oviduct of the same female, both the anterior and the pos- 

 terior rudiments have made considerable steps in develop- 

 ment. 



In sections taken from the hinder part of the body I found 

 that the posterior rudiments nearly agreed in size with those 

 in fig. D b. 



It is, however, still less easy than there to trace the junc- 

 tion of the posterior rudiments with the spinal cord, and the 

 upper ends of the rudiments of the two sides do not nearly 

 meet. 



In a considerable series of sections I failed to find any case 



