76 



The Journal of Heredity 



chromosomes will be bivalent and not 

 tetravalent in diakinesis or the hetero- 

 typic prophase. 



From a genetic point of view also it 

 is particularly desirable that the con- 

 ception of tetrads should not be intro- 

 duced where they do not exist. In 

 such cases the essential feature of 

 meiosis is the separation of the chromo- 

 some pairs, and the complications intro- 

 duced by the formation of tetrads and 

 the resulting possibilities in the distri- 

 bution of the four segments in the two 

 divisions, do not arise. 



Whether the pairing of chromosomes 

 is parasynaptic or telosynaptic is, 

 therefore, a matter of cytological detail 

 which need not fiecessarily affect the 

 distribution of the chromatic material. 

 But this is a subject into which I do 

 not propose to enter here. One must 

 expect to find differences in detail, and 

 even in important points as well, be- 

 tween the phenomena of meiosis as 

 observed in different organisms. The 

 time has passed when it is reasonable 

 to try to squeeze them all into one 

 mould. Since considerable attention 

 has been given recently to the cytolog- 

 ical basis of "crossing over" it may be 

 worth while referring to a few points 

 in this connection, in order to show that 

 some plants provide a cytological basis 

 for such phenomena while others do 

 not. 



It was recently shown that in Lac- 

 tuca' the chromosomes in early diaki- 

 nesis are twisted around each other, 

 and that this twist often persists as the 

 chromosome pairs condense. This 

 would lead to a breaking across the 

 twist when the pairs separate, in other 

 words to crossing over. Such twisted 

 pairs of chromosomes have often been 

 figured during diakinesis in various 

 plants, for example in Adoxa moschatel- 

 lina^ and in Smilacina.^ But I believe 

 the later consequences of the twist have 

 first been traced in Lactuca. In sharp 

 contrast to the twisting and probable 

 crossing over of chromosome segments 

 in Lactuca is the condition in Oeno- 

 thera, where such twisting has never 

 been seen and is not to be expected on 

 account of the short and stout shape of 

 the chromosomes.^ As is well known, 

 in Oenothera the chromosomes form a 

 chain end-to-end like a string of sau- 

 sages, and when they ultimately come 

 to be side-by-side in diakinesis they are 

 already in the short and stout condition 

 in which twisting about each other is 

 impossible. This is a very disturbing 

 fact for those who write about "crossing 

 over" in Oenothera, and obviously if 

 such crossing over occurs in Oenothera 

 the phenomena will have to find some 

 other explanation. 



1 Gates and Rees. 1921. A cytological study of pollen development in Lactuca. Annals of 

 Botany 35:365-398, pis 4. 



^ Lagerberg, T. 1909. Studien iiber die Entwicklungsgeschichte und systematische Stellung 

 von Adoxa moschatellina L. Kungl. Svensk. Vet. Akad. Handlingar, Vol. 44, No. 4. 



3 Lawson, A. A. 1912. A study in chromosome reduction. Trans. Roy. Soc. Edin. 48: 601-627. 

 * See Gates, R.R. 1908. A study of reduction in Oenothera ruhrinervis. Bot. Gazette 46: 1-35. 



