ORIGIN OF THE TETRADS 257 



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

 number of observers have maintained that reduction may be effected 

 by the union or conjugation of chromosomes that were previously 

 separate. This view agrees in princii)le with that of Ruckert, 

 Hacker, and Vom Rath ; for the bivalent chromosomes assumed by 

 these authors may be conceived as two conjugated chromosomes, ft 

 seems to be confirmed by the observations of Born and Pick on 

 Amphibia and those of Ruckert on selachians {Pristiunis)\ for in all 

 these cases the number of chromatin-masses at the time the first 

 polar body is formed is but half the number obser\^ed in younger 

 stages of the germinal vesicle. In Pristinrus 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 Caloptenus the spireme of the first sj^erma- 

 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 Cryllotalpa} 



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



earthworm {Linnbriciis terrestris), as described by Calkins ('95, 2), 



whose w^ork was done under my own direction. Calkins finds that 



the spireme splits longitudinally and then divides transverselv 



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 huigitudinally, 



a b 1 , r ^ r , . a b a\x ^, , 



I.e. -, etc., and the formula for a tetrad is I , or — i— Such 



a b a\b 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 Pristinnis. 



While these observations are not absolutely conclusive, they never- 



^ Montgomery, who has denied the occurrence of a longitudinal division in roitotoma 

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

 Euchistis ('99). 



