THE REDUCTION OF Till-: CHROMOSOMES 245 



mitosis and separate into two haploid groups at the second. To this 



simple form of reduction Goldschmidl applied the term " Primsertypus." 



Gregoire (1909a), on the contrary, found parasynapsis and the usual 

 mode of reduction in Zodgonus. 



The interpretation at one time given by Ruckeri (1893, 1894 

 Haecker (1895), and vom Rath (1895) for certain copepoda is shown in 

 Fig. 95, E. The continuous spireme splits throughoul its length and then 

 breaks into the haploid number of segments. These again break trans- 

 versely, forming chromosome tetrads, each composed of two split chromo- 

 somes arranged end-to-end. In some species the chromatids open oul to 

 form four-parted rings, whereas in others they maintain the rod form. 

 A separation occurs along the line of the original split at the first mitosis, 

 which is therefore equational, and along the plane of conjugation ai the 

 second mitosis, which is therefore reductions!. In Dicroccelium Gold- 

 schmidt (1908) reported that such tetrads divide reductionally at the 

 first mitosis. Lerat (1905), moreover, has found that in Cyclops strenu 

 one of the forms used by the earlier workers, the tetrads arise by a parallel 

 conjugation of thin threads which later split. 



A third mode of tetrad behavior is that reported by Paulmier I 1899 

 for Anasa tristis and by Foot and Strobell (1905, 1907) for Anasa and 

 Allolobophora foetida (Fig. 95, F). Here the chromosomes conjugate 

 end-to-end, the bivalents so formed then splitting longitudinally, giving 

 tetrads which take on a cross or ring form. At the first mitosis the sepa- 

 ration is along the plane of conjugation, effecting reduction, and at the 

 second it is along the plane of splitting. According to McClung 1 1902 

 Sutton (1902, 1905), Robertson (1908), and others, such tetrads separate 

 reductionally at the second mitosis (postreduction) rather than af the 

 first (prereduction) in certain orthopterans studied by them. 



Figure 96 illustrates the origin of chromosome tetrads of five charac- 

 teristic types by the two prominent modes of reduction described in 

 detail in foregoing pages. According to Scheme A iJ, Ci), two chromo- 

 somes conjugate parasynaptically while in the form of slender threads. 

 Instead of remaining unsplit as in most plants, each member then -pin- 

 longitudinally in a plane at right angles to the conjugation plane, thus 

 giving a tetrad composed of four parallel strands (chromatids l> . 

 According to Scheme B (A2-C2), the two chromosomes are at firsl ar- 

 ranged telosynaptically in the spireme and t he lat ter splits t hroughoul it- 

 length. The two conjugating members then take up a side-by-side 

 position, and their split, instead of becoming obscured as usually occurs 

 in plants, remains open, giving the tetrad of parallel strand- D 



The tetrad, by whichever met hod it has arisen, may now undergo a 

 variety of alterations, some of which are shown at /•.' and /'. Tin- chroma- 

 tids may simply shorten and t hicken, t he tel rad at diakinesis maintaining 

 the form of parallel rods (Ei,Fi). They may open out along the plane of 



