For each myotome there is a single nerve, which enters, as in the case of other fishes, the intermuscular septum. This point may easily be determined by means of longitudinal sections, or less easily from an examination of macerated specimens. I agree with Langerhans in denying the existence of ganglia on the roots of the nerves.
The present Monograph is a reprint of a series of papers published in the _Journal of Anatomy and Physiology_ during the years 1876, 1877 and 1878. The successive parts were struck off as they appeared, so that the earlier pages of the work were in print fully two years ago. I trust the reader will find in this fact a sufficient excuse for a certain want of coherence, which is I fear observable, as well as for the omission of references to several recent publications. The first and second chapters would not have appeared in their present form had I been acquainted, at the time of writing them, with the researches which have since been published, on the behaviour of the germinal vesicle and on the division of nuclei. I may also call attention to the valuable papers of Prof. His[66] on the formation of the layers in Elasmobranchii, and of Prof. Kowalevsky[67] on the development of Amphioxus, to both of which I would certainly have referred, had it been possible for me to do so.
Footnote 66: _Zeitschrift f. Anat. u. Entwicklungsgeschichte_, Bd. II.
Footnote 67: _Archiv f. Micr. Anat._ Bd. XIII.
Professor His deals mainly with the subjects treated of in Chapter III., and gives a description very similar to my own of the early stages of development. His interpretations of the observed changes are, however, very different from those at which I have arrived. Although this is not the place for a discussion of Prof. His's views, I may perhaps state that, in spite of the arguments he has brought forward in support of his position, I am still inclined to maintain the accuracy of my original account. The very striking paper on Amphioxus by Kowalevsky (the substance of which I understand to have been published in Russia at an earlier period) contains a confirmation of the views expressed in chapter VI. on the development of the mesoblast, and must be regarded as affording a conclusive demonstration, that in the case of Vertebrata the mesoblast has primitively the form of a pair of diverticula from the walls of the archenteron.