116 FUNDAMENTALS OF CYTOLOGY 



of fruiting in mixed cultures, and especially the regular pair-by-pair luiion 

 of nuclei of two strains in the multinucleate reproductive organs of certain 

 fungi. Here again we have valuable suggestions but no complete 

 explanation of the important feature we wish to understand. 



What causes diplotene opening? The fact that the synaptic mates in 

 the tetrad tend to separate soon after each of them becomes visibly double 

 suggests a causal connection between these two events. The hypothesis 

 that single threads attract while double ones repel one another has been 

 prominent in cytology for some time. At present it appears doubtful 

 that singleness in itself is a decisive factor in bringing homologues into 

 proximity, since doubled homologues also move together in some instances. 

 Moreover, it is visible doubling only, and probably not actual doubling, 

 that immediately precedes the opening of the chromatids at diplotene. 

 With regard to repulsions, it seems more likely that they are an important 

 factor, not only in diplotene opening, but also in the spacing of the 

 chromosomes at diakinesis and metaphase and in the widening out of 

 associated chromatids at anaphase I. 



Does coiling have a role in meiosis? In meiosis, as in somatic nuclear 

 cycles, coiling makes it possible for long chromonemata to be carried in 

 chromosomes of compact form during their segregation into daughter 

 groups. A special meiotic role of factors influencing coiling is indicated 

 by the complete uncoiling and great attenuation of the chromonemata in 

 the leptotene stage. The hypothesis has been advanced that it is just 

 this condition, brought on by a stronger or more prolonged action of 

 certain physiological factors, that makes complete synapsis possible, the 

 chromonemata at this one stage in the life cycle being in a condition 

 permitting forces acting over short distances between individual pairs of 

 homologous units to bring two long series of such units into close asso- 

 ciation. The physical causes of coiling are being sought in order to 

 improve our understanding of the chromosomal changes from diplotene 

 onward, notably shortening and chiasma terminalization, and to gain 

 insight into the molecular architecture and growth of the chromonemata. 



What is the mechanism of crossing over? That two of the four 

 chromatids actually do exchange corresponding portions, in all probability 

 at a chiasma, has been proved with heteromorphic homologues, i.e., 

 homologous chromosomes differing here and there in certain visible fea- 

 tures which make it possible to identify particular regions of the chromo- 

 somes before and after the exchange. How the exchange is actually 

 accomplished is still an unsolved problem. It is known that breakage of 

 chromonemata may be induced by such agencies as X rays and that 

 under certain circumstances freshly broken ends tend to unite in new 

 patterns. It is thei-efore a logical assumption that something of the kind 

 may occur naturally in crossing over, the precision with which correspond- 



