THE REDUCTION OF THE CHROMOSOMES 



241 



telophase, and after being temporarily obscured functions in the post- 

 homceotypic division. 1 



A variation of Scheme B has been observed in (Enothera (Gates 1908, 

 1909, 1911; Geerts 1908; B. M. Davis 1909, 1910, 1911); in Fucus and 

 Cutleria (Yamanouchi 1909, 1912) ; in Bufo (King 1907) ; and in a few 

 other forms. Here the spireme in the heterotypic prophase does not 

 become double, the split for the second division appearing first in the 

 heterotypic anaphase. 



Comparison of Schemes A and B. According to both of the fore- 

 going prominent theories of reduction the conjugated chromosomes 

 separate at the first maturation mitosis, thus causing reduction, and 



v@o 



v 



A (!S 

 V (il*. 



FIG. 93. -Diagram showing distinction between Schemes A and B. See text. 



divide longitudinally (equationally) at the second mitosis, so that the 

 final result is essentially the same: two of the resulting four nuclei differ 

 qualitatively from the other two in their chromatin content (Fig. 93). 

 The distinction between the two interpretations is nevertheless an im- 

 portant one, and may be emphasized in the following summary. 



According to Scheme A the double character of the chromatin spiremes 

 of the early heterotypic prophase is due to a lateral pairing of simple 

 threads each representing an entire somatic chromosome, the second con- 

 traction not being significant as regards pairing. The bivalent chromo- 

 somes so formed, after much shortening and thickening, are separated in 

 the heterotypic mitosis, during the anaphase of which (or earlier in the 



1 A more detailed summary of this view may be found in a review of Miss Digby's 

 paper on Osmunda by the present author (1920a). 



16 



