PREFACE TO THE SECOND EDITION 



Since the appearance of the first edition of this work, in 1896, the 

 aspect of some of the most important questions with which it deals 

 has materially changed, most notably in case of those that are 

 focussed in the centrosome and involve the phenomena of cell-division 

 and fertiUzation. This has necessitated a complete revision of the 

 book, many sections having been entirely rewritten, while minor 

 changes have been made on almost every page. 



In its first form, the work was compressed within limits too nar- 

 row for a sufficiently critical treatment of many disputed subjects. It 

 has therefore been considerably enlarged, and upwards of fifty new 

 illustrations have been added. The endeavour has, however, still been 

 made to keep the book within moderate limits, even at some cost of 

 comprehensiveness ; and the present edition aims no more than did 

 the first to cover the whole vast field of cellular biology. Its hmita- 

 tions are, as before, especially apparent in the field of botanical 

 cytology. Here progress has been so rapid that, apart from the dif- 

 ficulty experienced by a zoologist in the attempt to maintain a due 

 sense of proportion in reviewing the subject, an adequate treatment 

 would have required a separate volume. I have therefore, for the 

 most part, considered the cytology of plants in an incidental way, 

 endeavouring only to bring the more important phenomena into rela- 

 tion with those more fully considered in the case of animals. 



The steady and rapid expansion of the literature of the general 

 subject renders increasingly difficult the historical form of treatment 

 and the citation of specific authority in matters of detail. This plan 

 has nevertheless still been followed as far as possible, despite the 

 increased bulk of the book and the encumbrance of the text with 

 references thus occasioned, in the hope that these disadvantages will 

 be outweighed by increased usefulness of the work. I beg the 

 reader to remember, however, that no approach to a complete history 

 of cytology and experimental embryology could be attempted, save 

 in a work of far greater proportions, and that it has been necessary 



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