252 



J^EDCCTIOX OF THE CHROMOSOMES 



to form a tetrad ?;roup, and the eleven tetrads then place themselves 

 in the equator of the spindle for the first ]H)lar body (Fig. 123, 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)^ 



Fig. 125. —Germinal vesicles of various eggs, showing chromosomes, tetrads, and nucleoli. 



A. A copepod {Hetcrocope) showing eight of the si.xteen ring-shaped tetrads and the nucleo- 

 lus. [Rl'ckert.] 



B. Later stage of the same, condensation and segmentation of the rings. [RUCKERT.] 



C. " Cyclops strenuus," illustrating Hacker's account of the tetrad-formation from elongate 

 double rods ; a group of " accessory nucleoli." [Hacker.] 



D. Germinal vesicle of an annelid {Op/iryotroc/ia) showing nucleolus and four chromosomes. 

 [KORSCHELT.] 



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 chro- 

 mosomes in the egg. Paulmier's work on A?iasa and other Hemip- 

 tera ('99) gives the same result as the above in regard to the origin 

 of the tetrads (Figs. 126, 127). The process is, however, slightly 

 complicated by the fact that no continuous spireme-thread is formed, 

 while the rings are often bent or twisted and never open out to a 



