DEVELOPMENT OF THK ELASMOBRANCH FISHES. 89 



time when only two visceral clefts are present. Before the 

 appearance of a third visceral cleft in a part of the innermost 

 layer of each protovertebrae (which may be called the splanchnic 

 layer, from its being continuous with the mesoblast of _the^ 

 splanchnopleure), opposite the bottom of the neural tube, some 

 of the cells commence to become distinguishable from the rest, 

 and to form a separate mass. This mass becomes much more 

 distinct a little later, its cells being characterised by being 

 spindle-shaped, and having an elongated nucleus which be- 

 comes deeply stained by reagents (PI. 4, fig. \\,mp'}. Coin- 

 cidently with its appearance the young Dog-fish commences 

 spontaneously to move rapidly from side to side with a kind 

 of serpentine motion, so that, even if I had not traced the 

 development of this differentiated mass of cells till it becomes 

 a band of muscles close to the notochord, I should have had 

 little doubt of its muscular nature. It is indicated in figs, n, 

 12, 13, by the letters mp'. Its early appearance is most pro- 

 bably to be looked upon as an adaptation consequent upon the 

 respiratory requirements of the young Dog-fish necessitating 

 movements within the egg. 



Shortly after this date, at a period when three visceral clefts 

 are present, I have detected the first traces of the spinal nerves. 

 At this time they appear in sections as small elliptical masses 

 of cells, entirely independent of the protovertebrae, and closely 

 applied to the upper and outer corners of the involuted epiblast 

 of the neural canal (PI. 4, fig. n t spn}. These bodies are far 

 removed from any mesoblastic structures, and at the same time 

 the cells composing them are not similar to the cells composing 

 the walls of the neural canal, and are not attached to these, 

 though lying in contact with them. I have not, therefore, suf- 

 ficient evidence at present to enable me to say with any cer- 

 tainty where the spinal nerves are derived from in the Dog-fish. 

 They may be derived from the involuted epiblast of the neural 

 canal, and, indeed, this is the most natural interpretation of 

 their position. 



On the other hand, it is possible that they are formed from 

 wandering cells of the mesoblast a possibility which, with our 

 present knowledge of wandering cells, must not be thrown aside 

 as altogether improbable. 



