256 KEDL'CTIOX OF THE CHROMOSOMES 



double fission of the chromatin-granules. If this be correct, there 

 can be no reduction in Weismann's sense ; for the four products of 

 each primary chromatin-granule are equally distributed among the 

 four daughter-cells. A similar conclusion, based on much more 

 incomplete evidence, was reached by Hrauer ('92) in the phyllopod 

 BiiDicliipus. 



Brauer's evidently conscientious figures very strongly sustain his 

 conclusion, which, reinforced by the earlier work of Hertwig and 

 Boveri, has until now seemed to rest upon an unassailable basis. 

 The recent work of Sabaschnikoff (97) nevertheless raises the possi- 

 bility of a different interpretation. Brauer himself justly urges that 

 the essence of the process lies in the double fission of the chromatin- 

 granules to which the formation of chromosomes is secondary.^ 

 Everything, therefore, turns on the manner in which the quadruple 

 granules arise ; and Sabaschnikoff's work gives some ground for the 

 view that they may arise, not by a double fission, but in some other 

 wa}'. 



According to this author there is a period (in the oogenesis) at which the nuclear 

 threads wholly disappear, the entire chromatin being broken up into granules. 

 From this state the granules emerge in quadruple form to arrange themselves in the 

 doubly split spireme exactly tis Brauer describes : and a few observations are given 

 (regarding the size and arrangement of the granules) which suggest the possibility 

 that the quadruple granules may arise by the conjugation either of four separate 

 granules or of two pairs of double granules. Since there is ground for the view that 

 tetrads may arise by the conjugation of chromosomes (see following section), there 

 is no a priori objection to such a conclusion. Could it be sustained, the maturation- 

 divisions of Ascaris would in fact involve a true reduction in Weismann's sense; 

 for despite the fact that the chromosomes are only longitudinally divided, the four 

 longitudinal constituents of each tetrad would not be equivalent with respect to the 

 granules, and it is the reduction of the latter (" ids") that forms the essence of Weis- 

 mann's hypothesis (p. 245). Another consideration, suggested to me by Professor 

 T. H. Morgan, opens still another possibility, which seems well worthy of test by 

 further research. As already stated (p. 88). the long chromosomes oi Ascaris are 

 plurivalent. since in all but the germ-cells each breaks up into a much larger number 

 of smaller chromosomes (Fig. 73. p. 148). If. therefore, the latter correspond to 

 the chromosomes of other forms in which tetrads occur (e.^. Cyclops or Ariei/iia), 

 the so-called ■• tetrad "' of Ascaris is a compound body : and the true process of 

 reduction must be sought in the origin of the smaller elements of which it is com- 

 posed, which are. perhaps, directly comparable with Sabaschnikoff's -granules." 

 Until the questions thus opened have been further studied, the case {or Ascaris must 

 remain open : and it is perhaps worth suggesting that a new point of view may here 

 be found for further study also of reduction in the vertebrates. - 



1 Cf. p. 113. 



- Bodies closely resembling tetrads are sometimes formed in mitosis, where no reduction 

 should occur. Thus, R. Hertwig ('95) has observed tetrads in the first cleavage-spindle of 

 echinoderm-eggs after treatment with dilute poisons (p. 306). Klinckowstrom figures them 

 in the sfcofni polar spindle of ProstheceriCiis eggs, while Moore ('95) describes in the elasmo- 

 branchs small ring-shaped chromosomes, not only in the first but also in the second sperma- 

 tocyte-divisions, concluding that no reduction occurs in either division. 



