200 REDUCTION OF THE CHROMOSOMES 



somes assumed by these authors may be conceived as two conjugated 

 chromosomes. It seems to be confirmed by the observations of Born 

 and Fick on amphibia and those of Riickert on selachians (Pristi- 

 urus) ; for in all these cases the number of chromatin-masses at the 

 time the first polar body is formed is but half the number observed 

 in younger stages of the germinal vesicle. In Pristiums there are 

 at first thirty-six double segments in the germinal vesicle. At a later 

 period these give rise to a close spireme, which then becomes more 

 open, and is found to form a double thread segmented into eighteen 

 double segments ; i.e. the reduced number. In this case, therefore, 

 the preliminary pseudo-reduction is almost certainly effected by the 

 union of the original thirty-six double chromosomes, two by two. 

 The most specific accounts of such a mode of origin have, however, 

 been given by Calkins (earthworm) and Wilcox (grasshopper). The 

 latter author asserts ('95) that in Caloptenus the spireme of the first 

 spermatocyte first segments into the normal number (twelve) of dumb- 

 bell-shaped segments, which then become associated in pairs to form 

 six tetrads. Each of these dumb-bell-shaped bodies is assumed to be 

 a bivalent chromosome, and the tetrad-formation is therefore inter- 

 preted as follows : 

 abed I ab-cd-kl a_\b ,]/ 



(spireme) (segmented spireme) c \ d g\ll 



There is, therefore, no longitudinal splitting of the chromosomes. 

 A careful examination of the figures does not convince me of the 

 correctness of this conclusion, which is, moreover, inconsistent with 

 itself on Wilcox's own interpretation. Since each germ-nucleus 

 receives six chromosomes, the somatic number must be 12, and 

 Wilcox has observed this number in the divisions of the sperma- 

 togonia. The 12 dumb-bell-shaped primary segments must there- 

 fore represent single chromosomes, not bivalent ones, as Wilcox 



assumes, and his primary tetrad must therefore be not - j^ as he 



assumes, but either - or (if we assume that the normal number of 



i 

 chromosomes undergoes a preliminary doubling) ^~~- Until this 



contradiction is cleared up Wilcox's results must be received with 

 considerable scepticism. 



The second case, which is perhaps better founded, is that of the 

 earthworm (Lumbricus tcrrestris\ as described by Calkins ('95, 2), 

 whose work was done under my own direction. Calkins finds, in 

 accordance with all other spermatologists save Wilcox, that the 

 spireme-thread splits longitudinally and then divides transversely 

 into 32 double segments. These then unite, two by two, to form 

 1 6 tetrads. The 32 primary double segments therefore represent 



