DIVERGENT ACCOUNTS OF REDUCTION 199 



the conjugation of infusoria, as already described (p. 165), the original 

 nucleus divides several times before union, and only one of the result- 

 ing nuclei becomes the conjugating germ-nucleus, while the others 

 perish, like the polar bodies. The numerical correspondence be- 

 tween the rejected nuclei or "corpuscles de rebut" has already been 

 pointed out (p. 168). Hertwig could not count the chromosomes 

 with absolute certainty, yet he states ('89) that in Paramcecinm 

 cniidatnui, during the final division, the number of spindle-fibres 

 and of the corresponding chromatic elements is but 4-6, while in 

 the earlier divisions the number is approximately double this (8-9). 

 This observation makes it nearly certain that a numerical reduction 

 of chromosomes occurs in the Protozoa in a manner similar to that 

 of the higher forms ; but the reduction here appears to be deferred 

 until the final division. 1 In the gregarines Wolters('9i)has observed 

 the formation of an actual polar body as a small cell segmented off 

 from each of the two conjugating animals soon after their union ; 

 but the number of chromosomes was not determined. 



In the unicellular plants there are indications of a similar process, 

 but the few facts at our command indicate that the reduction may 

 here take place not before, but after, conjugation of the nuclei. Thus 

 in the dermids Clostcrium and Cosmarium, according to Klebahn 

 (Fig. 99), the nuclei first unite to. form a cleavage-nucleus, after which 

 the zygote divides into two. Each of the new nuclei now divides, 

 one of the products persisting as the permanent nucleus, while the 

 other degenerates and disappears. Chmielewski asserts that a similar 

 process occurs in Spirogyra. Although the numerical relations of 

 the chromosomes have not been determined in these cases, it appears 

 probable that the elimination of a nucleus in each cell is a process of 

 reduction occurring after fertilization. 



F. DIVERGENT ACCOUNTS OF REDUCTION 



We can only touch on a few of the accounts of reduction which 

 differ from both the modes already considered. Of these the most 

 interesting are observations which indicate the possibility of, 



i. The Formation of Tetrads by Conjugation 



A considerable number of observers have maintained that reduc- 

 tion may be effected by the union or conjugation of chromosomes 

 that were previously separate. This view agrees in principle with 

 that of Riickert, Hacker, and vom Rath ; for the bivalent chromo- 



1 Cf. Moore on the spermatogenesis of mammals, p. 201, 



