vi THE CHROMOSOMES 



to quote ' chapter and verse ' for each statement. I 

 mention this in apology for a certain amount of 

 dogmatism imposed by limitations of space. The 

 illustrations are essentially diagrammatic and designed 

 to illustrate principles rather than to serve as actual 

 illustrations of ceU-division in particular organisms. 



In any subject it is impossible to avoid technical 

 terms, but I have endeavoured to reduce them to a 

 minimum, and have explained in the text all-. those 

 which are not self-explanatory. Where a term occurs 

 for the first time and is defined it is printed in italics. 

 In the description of meiosis I have adopted the term 

 bivalent instead of tetrad as being far less likely to 

 cause confusion. Barbarous terms such as ' hetero- 

 typic ', ' homotypic ', ' preheterokinesis ' and ' posta- 

 crosyndesis ' have done more to frighten the general 

 biologist away from chromosome cytology than any- 

 thing else, and those authors who introduced them 

 must bear responsibility for the delayed integration 

 of cytology into general biological knowledge. Only 

 by a rigid avoidance of such terms can we hope to 

 link up the study of the nucleus with colloid 

 chemistry on the one hand and with animal and plant 

 breeding on the other. 



For those who find certain parts of the book diffi- 

 cult to understand, I would recommend the use of 

 some models, which can be constructed in a few 

 minutes out of soft copper wire or plasticine, using the 

 illustrations as a guide. With the aid of these it 

 should be possible for anyone to understand the 

 details of chiasma -formation and meiosis. 



Department of Zoology, 



University College, London, 

 April 1937. 



