PREFACE.

In this volume we offer to the public the first part of what we hope may serve as a systematic introduction to the study of Embryology. Some apology is perhaps necessary for the separate publication of a part only of the whole subject; but we trust that the following reasons will justify the course we have adopted.

Those who have paid attention to recent embryological researches must be aware of what we may venture to call the tumultuous condition of many parts of the subject, and of the extreme difficulty in many cases of forming a clear and decided judgment without the aid of independent observations. It is this necessity of having repeatedly to work over contested points with a view to reconcile diametrically opposed statements, or to verify startling announcements, which has rendered so laborious the task we have undertaken, and which so much delays its completion.

On the other hand, whoever wishes to have a sound foundation of embryological knowledge cannot do better than gain a thorough insight into the development of the bird. The practical advantages offered by the hen's egg

altogether outweigh the theoretical objections to beginning with the avian type. In many respects, it might be thought desirable to commence with a holoblastic ovum; but the large food-yolk of the bird's egg is in many ways a great assistance to the study of changes going on in the blastoderm. The chick is of all embryos the best to begin with; when its history has once been mastered, the subsequent study of other forms becomes an easy matter.

We venture to hope therefore that we shall meet with general approval, in having described at considerable length the history of the chick, and in hastening the publication of our account, by bringing it forward in a separate form.

In the earlier chapters, especially, we have gone into very considerable detail; and in order to make the account intelligible to the beginner, have not been deterred by the fear of wearying our readers with elementary and recapitulatory statements. Debated matters and details of minor importance have been put in small print; these may be omitted by the student in reading the book for the first time. Though we have sometimes introduced names in connection with important observations, we have not thought it necessary to do this systematically. For recent or debated statements however, the authorities are always cited.

The worth of such a book as this will be very small if the student simply contents himself with reading what is written; and to facilitate the only really useful mode of study, that of actual observation, a few practical instructions have been added in an appendix.

The readiness with which the development of the skull can be studied in the chick renders it, in spite of obvious

objections, a suitable introduction to the important subject of cranial morphology. It is with this view that we have given a separate chapter on the skull, which we hope may serve as an introduction to the study of Mr Parker's elaborate memoirs.

In the remaining parts, which we shall do our best to complete as soon as possible, the several histories will be treated with much greater brevity, and much more space will be given to theoretical considerations.

The figures, whose source is not acknowledged in the text, were drawn by Miss A. B. Balfour, except a few by ourselves.

The drawing on wood was executed partly by Mr Allchin, but chiefly by Mr Collings; and all the drawings were cut by Mr Cooper. We have to thank those gentlemen for the trouble they have taken in a matter in which, for many reasons, the result never seems commensurate with the labour. We are much indebted to Professor Huxley for having kindly looked over the proofs of the Chapter on the Skull.

The work took its origin in a course of lectures delivered by myself, but many causes prevented my taking the task seriously in hand, until I was joined by my friend and former pupil Mr F. M. Balfour, whose share in the matter has, to say the least, been no less than my own.

M. FOSTER.