g3 CELL-DIVISIOX 



while in the other form (" variety iinivalens ") there is but one. Braiier 

 discovered a similar fact in the phyllopod Artcmia, the number of 

 somatic chromosomes being i68 in some individuals, in others only 



84 (p. 281). 



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 plnrivaloit for such chromatin-rods.^ • The truth 

 of this view, which originated with Vom Rath, is, I think, conclusively 

 shown by the case of Artcniia described at page 281, and by many facts 

 in the maturation of the germ-cells hereafter considered. 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 page 148, 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 

 liunbricoides, where the normal number is 24. 



Hacker has called attention to the striking fact that plurivalent 

 mitosis is very often of the heterotypical form, as is very common 

 in the maturation-mitoses of many animals (Chapter V.), and often 

 occurs in the early cleavages of Ascaris ; but it is doubtful whether 

 this is a universal rule. 



3. Mitosis in the Unicellular Plants and Animals 



The process of mitosis in the one-celled plants and animals has a 

 peculiar interest, for it is here that we must look for indications of 

 its historical origin. But although traces of mitotic division were 

 seen in the Infusoria by Balbiani ('58-61), Stein ('59), and others 

 long before it was known in the higher forms, it has only recently 

 received adequate attention and is still imperfectly understood. 



Mitotic division has now been observed in many of the main divi- 

 sions of Protozoa and unicellular plants ; but in the present state of 



1 The words bivalent and uui-'alcni 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. 



