Talk:Book - Principles of Embryology (1956)

IN WRITING this book I have had three aims in mind. I have tried to expound a picture of embryology, which has been formed during a quarter of a century’s work, and to do so in a form sufficiently factual and systematic to be useful as a textbook for students specialising in that subject, or in the allied fields of genetics or experimental zoology. At the same time, I have attempted to meet the needs of research workers in other branches of biology who wish to find out what is going on in the study of development at the present time.

Embryology grew up as a branch of comparative anatomy; and when the science is referred to without qualification, even to-day most biologists probably think first of a descriptive account of developmental changes in anatomy and histology. But there is, of course, by now a very large body of data relating to the causal analysis of development. This is often regarded as a separate corpus of knowledge, referred to not as ‘embryology’ but as ‘experimental embryology’. A few decades ago, phylogeny and the evolutionary aspects of comparative anatomy constituted the core of animal biology, and it was not unjustified for the descriptive approach to development to be accorded the title ‘embryology’ tout simple. But now the situation seems to me to be different. The part of our subject which is of prime interest as a facet of general biology is that which deals with causal analysis, and if anyone claims to have studied embryology, this is the part which we ought to expect him to know about. I have therefore distributed the weight in this book in a manner quite different to that usual in textbooks of embryology, with more emphasis on the experimental and less on the descriptive approach. In fact, of the latter I have provided only the bare minimum which suffices to make the experimental work comprehensible. This book is, however, not intended to be for most students their first contact with embryology, but rather to serve the needs of their later university years; and it is to be expected that most users of it will have made some preliminary acquaintance with the anatomical facts, either in practical class work or through one of the many elementary texts which exist. Perhaps the ideal previous reading would be Barth’s excellent Embryology, which has the advantage of providing not only a fuller descriptive account, but also a very stimulating introduction to the experimental analysis.

In surveying such a wide field as embryology, within a compass that can be used as a text by students, a considerable amount of selection has to be exercised. It is natural, and indeed probably desirable, that an author should devote most attention to those aspects of the subject on which he has himself worked. I am conscious that I have given more space to the

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amphibia, birds and Drosophila, and less, say, to the echinoderms and the problems of fertilisation, than some other authors might have done. I think, however, that it is not merely a bee in my personal bonnet which has led me to include in the book a considerable discussion of topics which are conventionally counted as belonging to genetics. Embryology at the present time is in a betwixt-and-between state. It can no longer be wholly satisfied to operate in terms of the ‘complex components’ (such as organisers, fields and the like), which were discovered in the first successful experimental forays. On the other hand it is still too early to hope to find biochemical approaches which throw a general illumination on the scene. It is probably useful to try to formulate conceptional schemes in generalised chemical terms, such as those proposed by Weiss, or that discussed in Chapter XIX; but these must be recognised as no more than very abstract guides to possible directions which our thoughts may take, We have still to work through a region of facts and theories which deal with cellular constituents; and among this group of entities, which includes microsomes, mitochondria and such bodies, the genes (and possibly the plasmagenes) are certainly of crucial importance. It seems probable then that the most fundamental embryological theories of the immediate future will be phrased largely in terms of genes or of other bodies of a similar order of complexity; and in so far as this is true, no adequate discussion of embryology can be given without devoting a great deal of attention to the related aspects of genetics.

One of the difficulties in writing a book of this kind is to decide what references to literature should be provided. Anything approaching a complete bibliography would be too unwieldy. I have attempted two things; to provide an introduction to modern trends of work by giving a fairly large number of citations of recent papers whose results are being quoted; and to strike a balance between giving credit to the first discoverers of various facts and ideas, and indicating the most up-to-date summaries and reviews of the different topics. I can only beg the indulgence of any of my colleagues who may feel that I have cither overlooked their priority or failed to recognise the soundness of a recent summing-up. In any case, the bibliographic apparatus of such a book is inevitably a forest in which the student can only too easily lose himself. I have therefore, at the end of each chapter, given a very short selection of works which are suggested as valuable further reading, either to bring the student in contact with some of the original factual material, or to introduce him to some of the stimulating ideas which run parallel to, or even contradict, those advanced in the text.

Edinburgh, July 1954 C.H.W. Preface