Studies on Chromosomes. 401 



wholly fails to explain the origin or meaning of the accessory 

 chromosome, nor does it account for the surprising fact (of which 

 the phenomena m Brochymena seem to leave no doubt) that two 

 chromosomes may unite in synapsis, subsequently part company 

 so as to divide as univalents in the first mitosis, but again con- 

 jugate to form a bivalent m the second mitosis. It seems likely 

 that further comparative study of this phenomenon may throw 

 important light on the general mechanism of karyokinesis and 

 reduction. 



The history of the idiochromosomes possesses a more general 

 interest in the strong support that it lends to the general theory of 

 the individuality of chromosomes, to the specific conclusions of 

 Montgomery and Sutton in regard to synapsis, and especially to 

 the correlation of the phenomena of reduction with those of Men- 

 delian inheritance attempted by the last-named author ('02, '03). 

 It has been assumed by some authors, including some of those who 

 have accepted Montgomery's remarkable conclusion ('01, i, '04) 

 that corresponding paternal and maternal chromosomes unite in 

 synapsis, that in this process the individuality of the conjugating 

 chromosomes is completely lost — "Sie vereinigen sich zu einem 

 einzigen Zygosom, aus dem erst wieder zwei neue Chromosomen 

 hervorgehen."^ It is undoubtedly true that frequently all visible 

 traces of the duality of the bivalents that emerge from the synapsis 

 stage are for a time lost; and as Sutton suggested ('03, p. 243), such 

 cases as those of first crosses that breed true — and I may add, 

 perhaps also those in which blended inheritance or weakening of 

 dominance occurs — may be taken to indicate that a permanent 

 fusion, or intermixture of the chromosome-substances, may really 

 take place. But, on the other hand, the history of the idiochro- 

 mosomes in cases where they remain separate through the whole 

 growth-period leaves not the least doubt that as far as these 

 particular chromosomes are concerned the same two that unite in 

 synapsis persist as distinct individuals to be afterward separated 

 by the reducing division and assigned to different germ-cells. 

 This preliminary conjugation and subsequent separation ensures 

 that the germ-cells shall be "pure" in respect to these particular ; 

 chromosomes — /. e., that both shall not enter the same spermat- 

 ozoon — and if this be true of one pair of the conjugating chromo- ' 



'Strasburger, '04, p. 26; cf. also Bonnevie, '05. 



