THE REDUCTION OF THE CHROMOSOMES 245 



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

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

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

 mode of reduction in Zoogonus. 



The interpretation at one time given by Riickert (1893, 1894), 

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

 Fig. 95, E. The continuous spireme splits throughout 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 out 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 at the 

 second mitosis, which is therefore reductional. In Dicrocodium Gold- 

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

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

 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 (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 (1902), 

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

 reductionally at the second mitosis (postreduction) rather than at 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 (Ai-Ci), two chromo- 

 somes conjugate parasynaptically while in the form of slender threads. 

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

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

 giving a tetrad composed of four parallel strands (chromatids) (D). 

 According to Scheme B (A z -Cz), the two chromosomes are at first ar- 

 ranged telosynaptically in the spireme and the latter splits throughout its 

 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 strands (D). 



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

 variety of alterations, some of which are shown at E and F. The chroma- 

 tids may simply shorten and thicken, the tetrad at diakinesis maintaining 

 the form of parallel rods (Ei, FI). They may open out alongjhe plane of 



