i 4 2 BOTANY 



species, and as is well known they are doubled at every act of fertilisation and again re- 

 duced at some definite stage which intervenes between this act and the formation of new 

 germ cells. But several instances are known in which the chromosomes appear to be- 

 have differently as regards their numerical constancy. Thus in the hybrid races of 

 Pritmila Kewensis and in certain cenothera mutants the numbers normal for the parents 

 are doubled, and in this duplicated fashion the corresponding numbers are retained 

 during the succeeding generations. Moreover when again crossed with forms typical as 

 regards the nuclear constituents the oenotheras at least behave in a manner different 

 from that which the investigations of Rosenberg on Dorsera would lead one to expect. 

 For at meiosis the chromosome number does not correspond with, but is as nearly as may 

 be intermediate between, the two forms which are numerically different, as Gates, 

 Geerts, and others have shown. 



Opinions still differ widely as to how the chromosomes derived from the one parent 

 normally pair with those originating from the other at meiosis, although nearly all in- 

 vestigators are united in concluding that such union actually does occur. There is 

 almost overwhelming evidence to show that the chromosomes retain their physiological 

 identity through the so-called resting periods of the nucleus, though they cannot then 

 be structurally distinguished by any methods at present known to us. Furthermore 

 that even during this period they have already passed through changes which culminate 

 in their longitudinal fission at the next mitosis. It is not possible at present to explain 

 at all fully either the meaning of this, or the mechanism by which it is effected. Former- 

 ly it was held that the chromosomes might themselves represent or be directly respons- 

 ible for the production of the distinctive character of a species or variety. Further 

 investigation has shown this to have been too crude a view, and although the chro- 

 mosomes are almost unquestionably associated very intimately with the transmission of 

 hereditary qualities, their precise relationln this respect still remains to be discovered. 

 The principal evidence on which their claim to importance in the hereditary transmission 

 of characters depends consists in (i) the exactitude with which the chromosomes are 

 distributed between the sexual cells (and also between the cells of the whole body), (2) 

 the probability, almost amounting to certainty, of their physiological permanence 

 throughout the life cycle of an organism, (3) the paired arrangement which subsists 

 between the chromosomes derived from the maternal and paternal parent respectively 

 within the cells of the offspring, and the alternative distribution of the individuals of 

 each pair between the sex cells to which this offspring may give rise, (4) the general 

 harmony which exists between the results of such distribution and those observed to 

 obtain in the breeding of hybrids and their descendants, (5) the remarkable discoveries 

 by zoologists, and especially by E. B. Wilson on the existence of an " odd chromosome " 

 and its distribution at meiosis among the germ cells. It is almost impossible to escape 

 the conclusion that in this " odd chromosome " (to which many names have been given) 

 we are witnessing the agent that is concerned in the determination of sex. 



Nemec and others have investigated the effects of drugs on dividing nuclei. Chloral, 

 for example, interferes with the completion of the process, and division of the nuclei may 

 not be followed by division of the cytoplasm. When, as often happens, the nuclei re- 

 sulting from this imperfect division subsequently fuse with one another again, the large 

 cells in which this has occurred can readily be recognised by their larger nuclei. In fact 

 the K-b relation insisted on by Hertwig is very obvious. Such cells may continue to 

 divide and contribute to the formation of the .tissue in which they lie, and they retain 

 their size characters as well as the increased number of chromosomes. Nemec further 

 showed that a corresponding increase of chromosomes might be produced in the haploid 

 gametophyte of Larix without influencing the prothalloid character of the cells. This 

 is, of course, in accordance with the observations made by Farmer and Digby on apos- 

 porous fern prothallia as well as with those of El. and Em. Marchal on mosses. The 

 enlarged cells tend to disappear during the further growth of the organ (commonly the 

 root) in which they were induced to form. Nmcc believes this to be effected partly by 

 suppression of cell division, and consequently obliteration, partly by a " heterotype " 



