MEWS IS 111 



Comparisons indicate that different species often show different character- 

 istic amounts of terminaUzation, some having almost none while others 

 show only terminal associations at the end of the prophase. 



Cytological and genetical evidence is as yet not sufficiently critical 

 to permit a confident decision between the two rival interpretations of the 

 chiasma. Various phenomena have been held to conform better to the 

 one-plane theory ^^ or to the two-plane theory, ^° but none of them can be 

 regarded as really decisive. What is needed is a series of observations of 

 such a nature that less dependence upon personal judgment will be 

 necessary. Direct evidence has been sought in the behavior of hetero- 

 morphic tetrads, in which the two chromatids of one synaptic mate 

 differ visibly from those of the other. In Aloe the opening is regularly 

 in the synaptic plane in the heteromorphic region (Belling, 19316). 

 Also, in strains of Zea having large terminal knobs on two of the four 

 chromatids the opening appears always to be in the synaptic plane at the 

 ends carrying the knobs. Since such knobs are masses of substance 

 extending beyond the ends of the other chromatids, and since they show a 

 decided tendency to stick together at various stages, it is quite possible 

 that they prevent an opening in the equational plane which might other- 

 wise occur. Tetrads with more minute grades of heteromorphism, 

 particularly in non-terminal regions, may be expected to yield more 

 trustworthy evidence on this point. But even here caution will be 

 necessary in determining along which plane a tetrad first opens in any 

 given region, because terminalization, if this occurs in the cases studied, 

 would separate regions of chromatids originally together and vice versa. 

 Favorable cytogenetic material should yield an answer to the question 

 of whether it is chromatid exchange, or terminalization, or both of these 

 processes, which reduces the number of chiasmata during the later por- 

 tion of the prophase, and to a considerable extent clarify our notions of 

 the mechanism of genetic recombination. 



Before proceeding further it will be well to point out the fact that 

 according to either the two-plane or the one-plane theory a tetrad with 

 one original chiasma and exchange of parts consists of two chromatids 

 altered by the exchange and two unaltered ones. Since these four 

 chromatids are distributed to four nuclei by the two meiotic mitoses, two 

 of these nuclei contain each a chromosome with its original coristitution, 

 while the other two contain each a chromosome made up of portions of the two 

 synaptic mates (Fig. 157). This is a point of cardinal importance with 

 respect to the discussions of genetical problems in subsequent chapters. 



2* Configurations in trivalent, quadrivalent, and catenated chromosomes (Belling, 

 1929; Darlington, 1930crf, 1931e), interlocked tetrads (Belling, 19316; Catcheside, 

 19316), aspects in pachynema stage (Gelei, 1921; Belling, 1931a). 



'" Genetic data in Drosophila (Sax, 1932; Beadle, 1932). See also S. Emerson and 

 Beadle (1932) and Beadle and Emerson (1932). 



