ORIGIN OF THE TETRADS 257 



{c) The Formation of Tetrads by Conjugation. — A considerable 

 number of observers have maintained that reductibn may be effected 

 by the union or conjugation of chromosomes that were previously 

 separate. This view agrees in principle with that of Riickert, 

 Hacker, and Vom Rath ; for the bivalent chromosomes 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 {Pristitirus); 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 Pristiurus 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 seg- 

 ments ; i.e. the reduced number. In this case, therefore, the prelimi- 

 nary 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 Caloptenns the spireme of the first sperma- 

 tocyte gives rise without longitudinal division to twenty-four chromo- 

 somes (double the somatic number). These then become associated 

 in pairs, and still later the twelve pairs conjugate two and two to form 

 six tetrads. There is, therefore, no longitudinal splitting of the chro- 

 mosomes. The a priori improbability of such a conclusion is in- 

 creased by the studies of Paulmier on the Hemiptera, which demon- 

 strate the occurrence of a longitudinal division in a number of these 

 forms and confirm the original studies of Vom Rath on Gryllotalpa} 



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

 earthworm {Lumbricns terrcstris), as described by Calkins ('95, 2), 

 whose work was done under my own direction. Calkins finds that 

 the spireme splits longitudinally and then divides transversely 

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

 16 tetrads. The 32 primary double segments therefore represent 

 chromosomes of the normal number that have split longitudinally, 



i.e. T' etc., and the formula for a tetrad is — 



at (^ 



b a\x _ , 

 , or — Such 

 /-' a ! X 



a tetrad, therefore, agrees as to its composition with the formulas 

 of Hacker, Vom Rath, and Ruckert, and agrees in mode of origin 

 with the process described by Ruckert in the eggs of Pristiurns. 

 While these observations are not absolutely conclusive, they never- 



1 Montgomery, who has denied the occurrence of a longitudinal division in Pentatotna 

 ('98, i), has subsequently found such a division in the nearly related if not identical genus 

 EuchiUis ('99). 

 s 



