VI PREFACE 



amphibia, birds and Drosophila, and less, say, to the echinoderms and the 

 problems of fertiHsation, 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 organ- 

 isers, fields and the hke), 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 

 generahsed 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 com- 

 plete bibhography would be too unwieldy. I have attempted two things; 

 to provide an introduction to modem 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 either overlooked their priority or 

 failed to recognise the soundness of a recent summing-up. In any case, the 

 bibhographic 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 valu- 

 able 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. 



