Section III 

 ZOOLOGY 



By E. W. MacBride, LL.D., D.Sc, F.R.S., etc.. 



Professor of Zoology, University of London, Imperial College of 

 Science and Technology, S. Kensington, 



and H. R. Hewer, A.R.C.S., D.I.C., M.Sc. 



Lecturer in Zoology, Imperial College of Science and Technology, 



S. Kensington. 



INTRODUCTORY 



The last ten years, and more particularly the last five, have 

 seen a very important change in the study of the minute structure 

 of the animal cell. In order to aj^preciate this fully it is necessary 

 to recapitulate briefly the earlier work. 



With the rediscovery of the elementary laws of Mendelian 

 ijiheritance at the beginning of the century, the previously 

 described nuclear changes acquired considerable significance. Not 

 only the influence of the Weissmanian germ plasm theory, but also 

 the knowledge that the main (if not only) part of the sperm 

 incorporated in the zygote was the nucleus, induced cytologists to 

 concentrate upon the nucleus in looking for the material basis of 

 inheritance. When the remarkable parallelism between the 

 behaviour of the chromosomes in the maturation divisions and the 

 segregation of the factors assumed by the Mendelists was realised in 

 broad outline, it became the aim and purpose of cytologists to 

 establish the identity of chromosomal material with these factors. 



The result of this was to concentrate attention almost entirely 

 on the nucleus, and at the same time to reduce the cytologist to 

 the position of an assistant to the geneticist. It was the function 

 of the cytoiogist to provide the material phenomena upon which 

 the geneticist could build his theories. As genetical research 



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