48 CYTOLOGY 



CHAP. 



(2) The Mutual Relations of the Homologous Chromosomes during Syndesis 



Three views are possible as to the mutual relations of the chromosomes 

 which unite in syndesis, and these views have, as a matter of fact, all 

 been upheld. They are : 



1. Syndesis is a temporary fusion of the conjugating chromosomes, 

 comparable to that of conjugating Infusoria, and the chromosomes 

 separate again in the diplotene stage. (Many cytologists — e.g. Schreiner.) 



2. The chromosomes fuse completely at syndesis, so that the two 

 chromosomes which separate in the diplotene nucleus are not the chromo- 

 somes which came together in the zygotene stage, but new products 

 formed by the longitudinal fission of the compound chromosome formed 

 in syndesis (Bonnevie, 1906 ; Vejdovsky, 1907, 1911 ; von Winiwarter 

 and Sainmont, 1909 ; Wilson, 1912). 



3. Syndesis is only a temporary approximation of homologous chromo- 

 somes, which never become organically continuous (Gregoire, 1910). 



The distinction between the last view and the other two is a real one, 

 but the difference between the first two is largely a matter of words. 

 If two chromosomes fuse, a mutual influence, if not actual exchange of 

 substance, is implied ; whether or not the chromosomes which separate 

 afterwards are to be considered as the same as those that entered into 

 combination is a question of dialectics. Are two separating exconjugant 

 Paramecia, after exchanging micronuclei, the same individuals as those 

 which entered into conjugation ? 



While there is therefore no need to attempt to decide between the 

 first two views, the question of whether organic continuity is or is not 

 established between the approximated chromosomes is a very important 

 one. As we shall see later, the possibiUty of an exchange of substance 

 between homologous chromosomes is an important — probably indeed 

 essential — supplementary hypothesis to the chromosome theory of 

 heredity. 



As in so many cases, however, it is very difficult to arrive at a decision 

 by direct observation. The view that the conjugating chromosomes do 

 not enter into organic continuity is founded upon certain cases in which 

 a dividing line between them appears to be present throughout the 

 whole period of syndesis. It therefore really rests on negative evidence, 

 namely, that at no time during syndesis is the dividing line absent— an 

 observation which, as any cytologist will recognize, would be an exceed- 

 ingly difficult one to establish. There is of course no need to suppose 

 that fusion takes place simultaneously over the whole chromosomes. 

 Indeed it is certain that in many forms it begins at one end and 

 spreads thence to the other. The presence of sUts here and there in 

 a zygotene chromosome is therefore no evidence that the constituents 

 are not fused at other points, or at other stages. 



