194 



REDUCTION OF THE CHROMOSOMES 



spermatogonium-clivision, which spHt lengthwise during the anaphase 

 and pass into the spermatocyte-nucleus without forming a reticulum. 

 Flemming had observed in 1887 that these daughter-chromosomes 

 split in the anaphase, but could not determine their further history. 

 Vom Rath found that each double daughter-chromosome breaks in 

 two at the apex to form a tetrad, which passes into the ensuing 

 spermatocyte without the intervention of a resting stage. ^ 



It is clear that in such cases the " pseudo-reduction " must take 

 place at an earlier period than the penultimate generation of cells. 

 In the salamander Flemming ('87) found that the " chromosomes " of 

 the spermatogonia appeared in the reduced number (twelve) in at least 

 three cell-generations preceding the penultimate. Vom Rath ('93) 



Fig. 97. — Longitudinal section through the ovary of the copepod Canthocamptus. [HACKER.] 



og. The youngest germ-cells or oogonia (dividing at ^^. -) ; a. upper part of the growth- 

 zone ; oc. oocyte, or growing ovarian egg ; ov. fully formed egg, with double chromatin-rods. 



traced the pseudo-reduction in both sexes back to much earlier stages, 

 not only in the larva;, but even in the embryo (!). This very remark- 

 able discovery showed that the pseudo-reduction miglit appear in the 

 early progenitors of the germ-cells during embryonic life — perhaps even 

 during the cleavage. This conjecture has apparently been substan- 

 tiated by Hacker ("95, 3), who finds that in Cyclops brevicornis the 



1 It is certain that these facts do not represent a universal type of maturation, for in 

 Ascaris there is no doubt that a true reticular resting stage occurs in the primary spermato- 

 cytes, and probably also in the germinal vesicle. Hacker found, moreover, that the same 

 species might show differences in this regard; for in Cyclops strenuus the first-laid eggs have 

 no resting stage, the double daughter-chromosome passing directly into the tetrads, while 

 in later broods of eggs a daughter-spireme, composed of long double threads, is formed. The 

 difference is believed by Hacker to be due to the fact that the earlier eggs are quickly laid, 

 while the later broods are long retained in the oviduct. 



