PREFACE 



For many years now it has been apparent to biologists that the nucleus 

 • — and especially its most conspicuous constituent, the chromatin — lias 

 special claims to be considered the master substance of the organism, 

 not only determining its form and activities within the life-time of each 

 individual, but by means of the gamete nuclei bringing about that organic 

 continuity between parent and offspring which we call heredity. The 

 absorbing interest of this conception has attracted many workers to the 

 field of nuclear cytology, and consequently the advance in knowledge 

 and the output of hypotheses have been very great in recent years. 

 As must happen in any growing branch of science, the results of all this 

 research are scattered through many pubHcations, and any one whose 

 duty it has been to lecture to students on this science must often have 

 felt difficulty in recommending to inquirers a concise course of reading 

 which will give a summary of the main results in this field of research, 

 and at the same time indicate where more detailed information can be 

 obtained. This book is an attempt to provide for this want. 



The discovery of mitosis, and the recognition of all that is implied 

 by that process, has led biologists to lay more stress on the nucleus than 

 upon the cell, considered as units, and lately the " unit " has tended to 

 shift to certain constituents of the nucleus, namely, the chromosomes ; 

 still more recently the advance of knowledge has made it profitable to 

 focus attention on the even more elementary " units " of which the 

 chromosomes themselves are composed. In this book, consequently, 

 problems concerning the organization and physiology of the cell as a 

 whole are scarcely touched upon, practically the whole space being 

 devoted to the nucleus and its constituent parts. Moreover, the field 

 has been still further restricted by confining attention chiefly to the 

 Metazoan nucleus. The nucleus of the Metaphyta is very similar, in 

 structure and behaviour, to the Metazoan nucleus, and only rarely needs 



4v rL 



