348 F. M. BALFOUR. 



the pleuro-peritoneal cavity extends primitively quite up to 

 the top of the protovertebrse ; and that thus a portion of a 

 sheet of mesoblast, at first perfectly continuous with the 

 splanchnic sheet from which is derived the muscular wall of 

 the alimentary canal, is converted into a part of the voluntary 

 muscular system of the body, having no connection whatever 

 with the involuntary muscular system of the digestive tract. 



The pleuro-peritoneal cavity is first distinctly formed at a 

 time when only two visceral clefts are present. Before the 

 appearance of a third visceral cleft in a part of the inner- 

 most layer of each protovertebrae (which may be called the 

 splanchnic layer^ from its being continuous with the meso- 

 blast of the splanchnopleure), opposite the bottom of the 

 neural tube, some of the cells commence to become distin- 

 guishable from the rest, and to form a separate mass. This 

 nciass becomes much more distinct a little later, its cells 

 being characterised by being spindle-shaped, and having an 

 elongated nucleus which becomes deeply stained by reagents 

 (PI. XV, fig. 11, m p') . Coincidently with its appearance th6 

 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 noto- 

 chord, I should have had little doubt of its muscular nature. 

 It is indicated in figs. 11, 12, 13, by the letters m p\ Its 

 early appearance is most probably to be looked upon as an 

 alaptation 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, XV, fig. 11, sp n). 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, sufficient evidence at present to enable 

 me to say with any certainty 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. 



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

 wandering cells of the mesoblast — a possibility which, with 



