482 



NATURE 



[June 17, 1920 



The Structure of the Nucleus. 



Cytology: With Special Reference to the Meta- 

 zoan Nucleus. By Prof. W, E. Agar. 

 Pp. xii 4-224. (London: Macmillan and Co., 

 Ltd;, 1920.) Price 12s. net. 



CYTOLOGY as a science is of relatively recent 

 growth, and its development was made pos- 

 sible only by improvements in the microscope. Yet 

 the study of the details of cell structure and 

 activity is now fundamental to almost every phase 

 of biology. Nevertheless, biologists are too fre- 

 quently content with a superficial or second-hand 

 acquaintance, with this fascinating field. 



Previous to 1900 the process of mitosis had 

 been investigated, the general constancy of the 

 chrohiosomes from one cell generation to another 

 was recognised, and their relations to fertilisation 

 and 'reduction were understood in a general way, 

 thrpu,gh the, studies particularly of Strasburger and 

 Hertwig : the foundations of the present cytology 

 were laid. .Synapsis had been recognised by 

 Moore as. a unique phase of the nucleus, and this 

 led to- a ■ decade of active research by Farmer, 

 Moore, Wilson, Strasburger, Gregoire, and a host 

 of other plant and animal cytologists to elucidate 

 the intricate course of events during the meiotic 

 or reduction , period. 



But it is during the present century that im- 

 provements in method and increasing accuracy 

 of observation have made possible the remarkable 

 series of discoveries which have led to the present 

 outlook on cytological problems. Particularly 

 noteworthy has been the intimate linking up of the 

 chromosomes with the problems of heredity, sex, 

 mutation, and morphogenesis. This era of work 

 began with the discovery of Montgomery in 1901 

 that the chromosomes in the nuclei of certain 

 animals were in pairs, the members of which are 

 respectively of maternal and paternal origin. 

 Sutton in 1903 clearly established this relationship 

 in an insect in which the chromosomes were of 

 different sizes, and it is now a commonplace of 

 observation in a large number of animals and 

 plants. Several writers had already, shortly before 

 the Mendelian phenomena became known, pointed 

 out the theoretical possibilities for heredity which 

 lie in the reduction division where the pairs of 

 chromosomes are separated to enter different germ- 

 cells. In the following two decades of research, 

 the chain of evidence connecting the chromosomes 

 with heredity and also with variation has drawn 

 ever closer, until now the relationship must be 

 regarded as definitely established. 



One of the earlier stages in this proof was the. 

 discovery of the sex chromosomes in insects and 

 NO. 2642, VOL. 105] 



afterwards in other animals. The initial sug- 

 gestion of McClung, linking a particular chromo- 

 some with sex, was followed by notable discoveries 

 by Wilson, Morgan, Doncaster, and a host of 

 other investigators. Then a constant relationship 

 between chromosome numbers and externa! 

 characters was shown in the case of the CEnothera 

 mutations; and still more recently the work of 

 Morgan and Bridges on the non-disjunction of 

 chromosomes and its relation to inheritance in 

 Drosophila, combined with an enormous contem- 

 porary accumulation of collateral evidence of many 

 kinds, has added the final link in the chain of 

 evidence. 



Recent work tends more and more not only to 

 analyse the chromosomes themselves into their 

 visible microscopic elements, but also to show the 

 relationships which these bear to the hereditary 

 Mendelian units. The chromatin morphology in 

 related species is also being compared, and even 

 chromosome phylogeny is no longer a visionary' 

 hypothesis. 



All these and other recent developments in the 

 field of cytology are admirably set forth in the 

 work before us by Prof. Agar, who has thereby 

 placed all biologists in his debt. The book will be 

 chiefly useful to cytologists as a summary of the 

 facts and the literature connected with a particular 

 field — the nucleus in animals above the Protozoa. 

 Within this purview are discussed many of the 

 problems connected with nuclear structure. The 

 author deals with mitosis and meiosis, syrigamy 

 and parthenogenesis, with a detailed discussion of 

 these phenomena in various animal forms. He 

 points out that the fundamental fact of meiosis is 

 the segregation of the members of each pair of 

 homologous chromosomes, and compares the 

 parasynaptic and telosynaptic methods of meiotic 

 pairing. We cannot agree with him that the 

 telosynaptic method has been definitely disposed 

 of, as it is still the best authenticated account in 

 a number of plants and animals. Nor can we find 

 any difiRculty in the belief that both methods may 

 exist in different organisms, a view first expressed 

 ten years ago. 



The account of meiosis in parthenogenetic eggs 

 is welcome, but the discussion of the germ track 

 in animals might have included a greater variety 

 of forms. 



The later chapters give a useful account of the 

 sex chromosomes, theories of chromosome 

 individuality and structure, and variations in 

 chromosome number. An interesting chapter on 

 heredity and morphogenesis considers the chromo- 

 some behaviour and sterility in hybrids, the cyto- 

 logical basis of mutation, and general questions 



