STUDIES ON CHROMOSOMES 417 



tion or with a process of reconstruction subsequent to synapsis. 

 I do not see how the analysis can be carried further without enter- 

 ing upon theoretical ground. Nevertheless, I do not hesitate to 

 accept the probability that the reduction-division, as thus defined, 

 involves a disjunction of chromatin-elements of some kind that 

 are involved in the production of the unit-factors of heredity 

 and that -the Mendelian disjunction may here find its explanation 

 (cf. De Vries, '03, Boveri, '04). It seems to me that the conclu- 

 sions indicated by Boveri several years ago ('04) still remain the 

 most probable ; that is to say, that the degree of union may vary 

 in different cases, involving sometimes no fusion (as is suggested 

 by the history of the XY-pair), sometimes complete fusion, in 

 other cases no more than a partial exchange of material. This 

 point will be again touched upon in connection with Janssens's 

 theory of the 'chiasmatype." 



The point that I wish here to emphasize is the validity of the 

 conceptions of bivalence and the reduction-division, which have 

 been more or less explicitly denied by a number of writers. I 

 accept, of course, the conclusion of Haecker ('07), Bonnevie ('08, 

 '11), Delia Valle ('07), Popoff ('08) and others, that neither the 

 heterotypical form of division nor an apparent tetrad-structure is 

 necessarily diagnostic of bivalence or a reduction-division. Tet- 

 rad-like chromosomes have been repeatedly described in somatic 

 divisions (see especially Delia Valle, Popoff, cited above), and also 

 in the univalent chromosomes of the second maturation-division 

 a striking example of which is the ' d-chromosome' of Nezara, 

 described in my seventh 'Study.' Bonnevie, especially, has 

 demonstrated in Nereis and other forms the very close similarity 

 of the somatic chromosomes (in the cleavage of the egg) to the 

 heterotype-rings of the maturation-divisions. Her conclusion is: 



Ich habe in meinen Objekten nicht nur fur die Annahme einer Reduk- 

 tionsteilung keinen einzigen Beweis finden konnen; meine Untersuchung 

 hat auch ergeben, dass die friiher in der heterotypischen Natur der ersten 

 Reifungsteilung gesehene Stiitze einer solchen Annahme sammtlich 

 hinfallig sind . . . Die beiden Reifungsteilungen miissen daher, 

 bis anderes bewiesen worden ist, als Aequationsteilungen aufgefasst 

 werden ('08, p. 271, '11, p. 241). 



