256 INTRODUCTION TO CYTOLOGY 



and Ascaris (Montgomery 1904, 1905, 1908, 1910), Alytes (Janssens 

 and Willems 1909), Helix and Sagitta (Stevens 1903; Ancel 1903), certain 

 Diptera (Stevens 1908, 1911), and Pediculus (Doncaster 1920). 



More recently it has been shown that the homologous chromosomes 

 may begin to show a paired arrangement even earlier in the cycle, in 

 some cases directly after the parental groups are brought together at 

 fertilization. In the Diptera, for example, Metz (1916a) has shown that 

 the association, which at certain stages is so close as to constitute a 

 synapsis, begins before the cleavage of the fertilized egg, and that the 

 paired condition is maintained in all cells, somatic and germinal, through- 

 out the life cycle. Metz examined 80 species and in all of them found 

 such a somatic pairing. In Culex (Stevens 1910, 1911; Taylor 1914, 

 1917), which has six chromosomes, the association can be seen in the 

 nuclei of the segmenting egg, and in the early larval stages there follows 

 an actual parasynaptic fusion, so that the somatic cells thereafter show 

 three bivalent chromosomes rather than six univalents. In the matura- 

 tion divisions the members of each pair separate, the gametes receiving 

 three chromosomes each, just as they would had conjugation begun in 

 the heterotypic prophase as usual. 



A loosely paired arrangement of the chromosomes in the somatic 

 cells of plants has been reported by Strasburger (1905, 1907, 1910) for 

 Galtonia candicans, Funkia Sieboldiana, Pisum sativum, Melandrium, 

 Mercurialis, and Cannabis; by Sykes (1908) for Hydrocharis, Lychnis, 

 Begonia, Funkia, and Pisum; by Overton (1909) for Calycanthus; by 

 Muller (1909, 1911) for Yucca and other forms; by Stomps (1910, 1911) 

 for Spinacia; by Kuwada (1910) for Oryza; by Tahara (1910) for Morus; 

 and by Ishikawa (1911) for Dahlia. This is another matter that will be 

 considered further in Chapter XII. 



The Nature of the Synaptic Union. — Because of the manner in which 

 chromosome behavior is at present being applied to the solution of the 

 problems of inheritance, no question concerning chromosome conjugation 

 is more important than that of the exact nature of the synaptic union. 

 In reviewing some of the opinions of this subject it will be convenient 

 to list separately the views of the telosynaptists and the parasynaptists. 



In such cases as those described by Henking and by Goldschmidt 

 Fig. 95, D) there is only a momentary end-to-end association of the fully 

 formed chromosomes on the spindle of the heterotypic mitosis, there 

 being no real fusion and almost no opportunity for an " interchange of 

 influences." In the other tetrad chromosomes formed by telosynapsis 

 (Fig. 95, E and F; Fig. 90) there is only slightly greater opportunity 

 for such interchange. According to Scheme B (Fig. 89) the synaptic 

 mates are at first arranged end-to-end, and only later, when partially 

 condensed, do they take up a. side-by-side position, allowing a more 

 intimate and extensive union for a short time. 



