IQO REDUCTION OF THE CHROMOSOMES 



first the separation of these two halves (corresponding with the origi- 

 nal longitudinal split), and second, the transverse division of each half, 

 the latter being the reducing division of Weismann. The number of 

 primary rods, from which the rings arise, is one-half the somatic 

 number. Hence each of them is conceived by vom Rath, Hacker, 

 and Riickert as bivalent or double ; i.e. as representing two chro- 

 mosomes united end to end. This appears with the greatest clear- 

 ness in the spermatogenesis of Gryllotalpa (Fig. 93). Here the 

 spireme-thread splits lengthwise before its segmentation into rods. 

 It then divides transversely to form six double rods (half the usual 

 number of chromosomes), which open out to form six closed rings. 

 These become small and thick, break each into four parts, and thus 

 give rise to six typical tetrads. An essentially similar account of the 

 ring-formation is given by vom Rath in Euclueta and Calaiiiis, and 

 by Riickert in Hcterocope and Diaptouius. 



That the foregoing interpretation of the rings is correct, is beauti- 

 fully demonstrated by the observations of Hacker, and especially of 

 Riickert, on a number of other copepods {Cyclops, Canthocamptus), 

 in which rings are not formed, since the splitting of the primary 

 chromatin-rods is complete. The origin of the tetrads has here been 

 traced with especial care in Cyclops strciiiiiis, by Riickert ('94), whose 

 observations, confirmed by Hacker, are quite as convincing as those 

 of Brauer on Ascaris, though they lead to a diametrically opposite 

 result. 



The normal number of chromosomes is here twenty-two. In the 

 germinal vesicle arise eleven threads, which split lengthwise (Fig. 94), 

 and finally shorten to form double rods, manifestly equivalent to the 

 closed rings of Diaptouius. Each of these now segments transversely 

 to form a tetrad group, and the eleven tetrads then place themselves 

 in the equator of the spindle for the first polar body (Fig. 94, C), in such 

 a manner that the longitudinal split is transverse to the axis of the 

 spindle. As the polar body is formed, the longitudinal halves of 

 the tetrad separate, and the formation of the first polar body is thus 

 demonstrated to be an "equal division" in Weismann's sense. The 

 eleven dyads remaining in the eggs now rotate (as in Ascaris), so that 

 the transverse division lies in the equatorial plane, and are halved 

 during the formation of the second polar body. The division is 

 accordingly a " reducing division," which leaves eleven single chromo- 

 somes in the egg, and it is a curious fact that this conclusion, which 

 apparently rests on irrefragable evidence, completely confirms Weis- 

 mann's earlier views, published in 1887,^ and contradicts the later 

 interpretation upheld in his book on the germ-plasm. 



1 Essay VI. 



