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                             THE WORKS

                                 OF

                     FRANCIS MAITLAND BALFOUR.

                             VOL. II.

                         Memorial Edition.




                            Cambridge:

               PRINTED BY C. J. CLAY, M.A. AND SON,
                     AT THE UNIVERSITY PRESS.




                         Memorial Edition.


                             THE WORKS

                                 OF

                     FRANCIS MAITLAND BALFOUR,

                        M.A., LL.D., F.R.S.,
                     FELLOW OF TRINITY COLLEGE,
      AND PROFESSOR OF ANIMAL MORPHOLOGY IN THE UNIVERSITY OF
                              CAMBRIDGE.


                             EDITED BY

                        M. FOSTER, F.R.S.,
      PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CAMBRIDGE;

                                AND

                       ADAM SEDGWICK, M.A.,
        FELLOW AND LECTURER OF TRINITY COLLEGE, CAMBRIDGE.


                             VOL. II.

               A TREATISE ON COMPARATIVE EMBRYOLOGY.

                       Vol. I. Invertebrata.


                              London:
                         MACMILLAN AND CO.
                               1885




          [_The Right of Translation is reserved._]




                             PREFACE.


My aim in writing this work has been to give such an account of the
development of animal forms as may prove useful both to students and
to those engaged in embryological research. The present volume, save
in the introductory chapters, is limited to a description of the
development of the Invertebrata: the second and concluding volume will
deal with the Vertebrata, and with the special histories of the
several organs.

Since the work is, I believe, with the exception of a small but useful
volume by Packard, the first attempt to deal in a complete manner with
the whole science of Embryology in its recent aspects, and since a
large portion of the matter contained in it is not to be found in the
ordinary text books, it appeared desirable to give unusually ample
references to original sources. I have accordingly placed at the end
of each chapter, or in some cases of each section of a chapter, a list
of the more important papers referring to the subject dealt with. The
papers in each list are numbered continuously, and are referred to in
the text by their numbers. These lists are reprinted as an appendix at
the end of each volume. It will of course be understood that they do
not profess to form a complete bibliography of the subject.

In order to facilitate the use of the work by students I have employed
two types. The more general parts of the work are printed in large
type; while a smaller type is used for much of the theoretical matter,
for the details of various special modes of development, for the
histories of the less important forms, and for controversial matter
generally. The student, especially when commencing his studies in
Embryology, may advantageously confine his attention to the matter in
the larger type; it is of course assumed that he already possesses a
competent knowledge of Comparative Anatomy.

Since the theory of evolution became accepted as an established
doctrine, the important bearings of Embryology on all morphological
views have been universally recognised: but the very vigour with which
this department of science has been pursued during the last few years
has led to the appearance of a large number of incomplete and
contradictory observations and theories; and to arrange these into
anything like an orderly and systematic exposition has been no easy
task. Many Embryologists will indeed probably hold that any attempt to
do so at the present time is premature, and therefore doomed to
failure. I must leave it to others to decide how far my effort has
been justified. That what I have written contains errors and
shortcomings is I fear only too certain, but I trust that those who
are most capable of detecting them will also be most charitable in
excusing them.

The work is fully illustrated, and most of the figures have been
especially engraved from original memoirs or from my own papers or
drawings by Mr Collings, who has spared no pains to render the
woodcuts as clear and intelligible as possible. I trust my
readers will not be disappointed with the results. The sources from
which the woodcuts are taken have been in all cases acknowledged, and
in the cases where no source is given the illustrations are my own.

I take this opportunity of acknowledging my great obligations to
Professors Agassiz, Huxley, Gegenbaur, Lankester, Turner, Koelliker,
and Claus, to Sir John Lubbock, Mr Moseley, and Mr P. H. Carpenter,
for the use of electrotypes of woodcuts from their works.

I am also under great obligations to numerous friends who have helped
me in various ways in the course of my labour. Professor Kleinenberg,
of Messina, has read through the whole of the proofs, and has made
numerous valuable criticisms. My friend and former pupil, Mr Adam
Sedgwick, has been of the greatest assistance to me in correcting the
proofs. I have had the benefit