All the nerves of Amphioxus, except the first pair, have their roots inserted in the dorsal part of the cord. In the case of the first two pairs the nerves of the two sides arise opposite each other; in the next few pairs, the nerves on the right-hand side gradually shift backwards: the remaining nerves spring alternately from the two sides of the cord. 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. X. A MONOGRAPH ON THE DEVELOPMENT OF ELASMOBRANCH FISHES. PUBLISHED 1878. PREFACE. 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.