Gates: Cytology and Genetics 



75 



table, nine out of ten kernels germi- 

 nated at an age of five days from the 

 time they were pollinated. The fact 

 that so many kernels germinated at 

 five days and the irregularity of the 

 later germinations indicate that an 

 occasional seed might be found which 

 would germinate at less than five days. 

 There is some question as to the 

 efTect of the kernels being allowed to 



remain on the spike as was done in this 

 experiment. The spikes dry very 

 rapidly at Aberdeen. It is thought 

 that no additional material enters the 

 seed from the spike for a period longer 

 than one hour after the culm has been 

 severed from the plant. Further ex- 

 periments are being conducted to 

 discover more of the details of germina- 

 tion at early ages. 



SOME POINTS ON THE RELA- 

 TION OF CYTOLOGY AND 

 GENETICS 



R. RuGGLES Gates 



University of London, Kings College 



IT IS necessary to point out certain 

 misstatements of fact made by 

 Miss E. Eleanor Carothers in her 

 review of Sharp's Cytology in the Jour- 

 nal OF Heredity (Vol. 12, p. 351). 

 This is not the place for a discussion of 

 detailed cytological points, and ref- 

 erence will only be made to certain 

 general features, particularly those hav- 

 ing a direct bearing on genetics. Miss 

 Carothers has rightly pointed out cer- 

 tain defects in the work in question 

 which are capable of being remedied in 

 a later edition. But she makes a serious 

 blunder in discussing the phenomena of 

 meiosis in plants, with which she evi- 

 dently has had little experience. She 

 refers to the statement of Sharp that in 

 diakinesis the heterotype chromosomes 

 contrast strikingly with the chromo- 

 somes in an ordinary somatic prophase, 

 whether they are secondarily split or 

 not. But she goes on to afiirm that 

 "this 'striking contrast,' however, de- 

 pends wholly upon the fact that the 

 chromosomes are secondarily split, that 

 is that they are tetrads and not dyads." 

 She also states that she "has had 

 enough contact with botanical material 

 to be confident that this statement is 

 true for plants as well as animals." 



This statement is particularly unfor- 

 tunate, because the understanding of 



meiosis as it occurs in most plants was 

 retarded for a decade by the search for 

 "tetrads" where they do not exist. It 

 is now well recognized by most plant 

 cytologists that in the majority of 

 plants tetrads do not occur, and that 

 the essential feature of the heterotypic 

 mitosis is the separation of pairs of 

 whole chromosomes. It is easy for such 

 pairs of chromosomes (gemini) to be 

 mistaken for tetrads through a super- 

 ficial examination, particularly if the 

 observer has the necessity for finding 

 "tetrads" already in mind. As regards 

 animals, there are many instances in 

 which clear tetrads have been observed, 

 but that they are certainly not uni- 

 versal is shown, for example, by the 

 history of the X Y chromosomes in 

 many forms, to take a single example. 

 Some of the more careful recent papers 

 on animal spermatogenesis discard the 

 conception of tetrads and speak of 

 bivalent chromosomes in diakinesis. 



The presence of tetrads merely indi- 

 cates that the paired chromosomes 

 have themselves split before the sepa- 

 ration of parts begins. Such a split is 

 necessarily precocious. When this 

 split occurs in the heterotypic anaphase 

 or telophase, as it does in Oenothera and 

 Lactuca among forms which I have 

 studied critically, then obviously the 



