STUDIES ON CHROMOSOMES 405 



Lastly may be mentioned the interesting facts observed in the 

 maturation of hybrids between parental forms having different 

 numbers of chromosomes. Well known as Rosenberg's results 

 on Drosera are ('04, '09), the main facts may be again outlined, 

 especially as exactly analogous results have recently been reached 

 by Geerts ('11) in hybrid Oenotheras. On crossing Drosera 

 longifolia (having forty chromosomes) with D. rotundifolia (hav- 

 ing twenty chromosomes) the hybrids have the intermediate 

 number of chromosomes, thirty (20 + 10). In the first matura- 

 tion-division appear ten double and ten single chromosomes, the 

 former undergoing a regular division and distribution to the poles, 

 while the latter fail to divide, undergo an irregular distribution, 

 and often fail to enter the daughter-nuclei. Rosenberg's inter- 

 pretation is that the ten rotundifolia chromosomes conjugate 

 with ten of the longifolia ones to form the ten bivalent (double) 

 chromosomes, leaving ten longifolia chromosomes as unpaired 

 univalents which undergo irregular distribution. The results of 

 Geerts are exactly analogous. Oenothera gigas -(twenty-eight 

 chromosomes) crossed with Oe. lata (fourteen chromosomes) 

 gives hybrids with twenty-one chromosomes (14 + 7). The 

 first division shows seven double (bivalent) and seven single 

 (univalent) chromosomes; and, as in Drosera, the bivalents divide 

 equally and symmetrically, while the univalents wander irregularly 

 along the spindle and often fail to enter the daughter-nuclei. 

 His interpretation is the same as that of Rosenberg. ^^ If cor- 

 rect, these results, indirect though they be, constitute almost an 

 experimental demonstration of both synapsis and reduction. 



In summing up, it is my opinion that in spite of all the apparent 

 contradictions and conflict of opinion concerning the modus oper- 



1^ Gates however ('09) in an earlier study of the same hybrid examined by 

 Geerts, was led to quite different results, concluding that half the pollen cells 

 receive 10 chromosomes and half 11. I can not, however, find evidence in his paper 

 to sustain his conclusion that " there is not here a pairing and separation of homolo- 

 gous chromosomes of maternal and paternal origin" (op. cit., p. 195). Gates gives 

 no account of the exact mode of distribution of the chromosomes in the hetero- 

 typic division, but only the end result. This result would however follow if ex- 

 actly such a pairing and disjunction took place as is described by Geerts, provided 

 the remaining chromosomes also underwent an approximately equal distribution. 

 It remains therefore to be seen whether the apparent contradiction of results is 

 real. 



