THE REDUCTION OF THE CHROMOSOMES 257 



Generally speaking, the parasynaptists have given more attention 

 to the details of the synaptic union than have the telosynaptists. Al- 

 though cases are on record in which there is only a momentary para- 

 synaptic association of fully formed chromosomes (von Voss 1914), 

 the association usually extends over considerable time. Most para- 

 synaptists hold that the conjugation begins with the association of the 

 leptotene threads before or during synizesis, continues through the 

 remainder of the prophase, and ends with the anaphasic separation 

 (Scheme A). The association of the synaptic mates is thus long and 

 intimate. Concerning the closeness of the union, however, opinions 

 differ widely. 



A few investigators (Vejdowsky 1907; Bonnevie 1906, 1908, 1911; 

 Winiwarter and Sainmont 1909: Schneider 1914) have thought that the 

 conjugating members fuse completely and lose their individual identity, 

 the "mixochromosome" so formed then undergoing two true longitudinal 

 splits along new planes at the two maturation divisions. In some cases 

 (Bonnevie; see p. 251) the fusion may not be fully consummated until 

 during the post-meiotic divisions. Others believe the split for the 

 heterotypic mitosis to be along the plane of conjugation (Cardiff 1906, 

 Fasten 1914, and others). Probably the most widely advocated view is 

 that there is no actual fusion of the synaptic threads, the latter main- 

 taining their identity completely. Although their association may at 

 times be so intimate that they seem to constitute a single thick thread, 

 the doubleness, if thus lost to view, reappears during later stages (Berghs 



1904, 1905; A. and K. E. Schreiner 1905, 1906; Mare"chal 1907; Overton 



1905, 1909; Robertson 1915, 1916) (Fig. 88). Several careful observers 

 have reported that the doubleness can be seen at all stages (Gregoire 

 1907, 1910; Schleip 1906, 1907; Montgomery 1911; Kornhauser 1914, 

 1915; Wenrich 1915, 1917). Gregoire, who has argued strongly for this 

 interpretation, has emphasized the ease with which the closely appressed 

 threads may be mistaken for a single thick structure. 



One of the most important suggestions which has been made concern- 

 ing chromosome conjugation is embodied in the "Chiasmatype Hypo- 

 thesis" of Janssens (1909). According to Janssens, the pairing threads, 

 though remaining separate throughout the greater part of their length, 

 fuse at one or more points as they twist about each other. When they 

 again separate a break occurs at each of these fusion points, but along a 

 new plane, so that each of the two resulting chromosomes is composed 

 of portions of both conjugating members (Fig. 149). This interpreta- 

 tion, which has been admitted as possible by several of the investigators 

 named in the preceding paragraph, is significant in that it shows how an 

 orderly evolution of chromosomes with new constitutions mayjoccur, a 

 point of great importance in connection with current conceptions of the 



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