MODIFICATIONS OF MITOSIS 6 1 



their ends. The ring finally breaks in two to form two U-shaped 

 chromosomes which diverge to opposite poles of the spindle as 

 usual. As will be shown in Chapter V., the divisions by which 

 the gernvcells are matured are in many cases of this type ; but 

 the primary rings here represent not two but four chromosomes, 

 into which they afterwards break up. 



3. Bivalent and Plurivalent Chromosomes 



The last paragraph leads to the consideration of certain varia- 

 tions in the number of the chromosomes. Boveri discovered that the 

 species Ascaris megalocephala comprises two varieties which differ in 

 no visible respect save in the number of chromosomes, the germ-nuclei 

 of one form (" variety bivalens " of Hertwig) having two chromo- 

 somes, while in the other form ("variety univalens") there is but one. 

 Brauer discovered a similar fact in the phyllopod Artemia, the 

 number of somatic chromosomes being 168 in some individuals, in 

 others only 84 (p. 205). 



It will appear hereafter that in some cases the primordial germ- 

 cells show only half the usual number of chromosomes, and in 

 Cyclops, the same is true, according to Hacker, of all the cells of 

 the early cleavage-stages. 



In all cases where the number of chromosomes is apparently 

 reduced ("pseudo-reduction " of Riickert) it is highly probable that 

 each chromatin-rod represents not one but two or more chromosomes 

 united together, and Hacker has accordingly proposed the terms 

 "bivalent" and " plurivalent " for such chromatin-rods. 1 The 

 truth of this view, which originated with vom Rath, is, I think, 

 conclusively shown by the case of Artemia described at p. 203, and 

 by many facts in the maturation of the germ-cells hereafter con- 

 sidered. In Ascaris we may regard the chromosomes of Hertwig's 

 "variety univalens" as really bivalent or double; i.e. equivalent 

 to two such chromosomes as appear in "variety bivalens." These 

 latter, however, are probably in their turn plurivalent, i.e. represent a 

 number of units of a lower order united together; for, as described at 

 p. i u, each of these normally breaks up in the somatic cells into a 

 large number of shorter chromosomes closely similar to those of the 

 related species Ascaris lunibricoidcs, where the normal number is 24. 



1 The words "bivalent" and "univalent" have been used in precisely the opposite sense 

 by Hertwig in the case of Ascaris, the former term being applied to that variety having two 

 chromosomes in the germ-cells, the latter to the variety with one. These terms certainly have 

 priority, but were applied only to a specific case. Hacker's use of the words, which is 

 strictly in accordance with their etymology, is too valuable for general descriptive purposes 

 to be rejected. 



