272 Transactions of the Society. 



distinguishable in size and shape, but the most important data 

 are derived by the study of multiple chromosmes in the 

 Orthoptera. Eeferring to the existence of a "hexad" in the 

 maturation division of a species with the accessory united 

 to an ordinary chromosome in the piemeiotic mitosis, McClung 

 comments thus : " One of the elements possesses a distinctive 

 character not shared by the others — it has an individual and 

 more or less independent movement which takes place at only 

 one time in all the history of the organism. Up to this one point 

 it is distributed in mitosis like the other chromosomes ; ... so much 

 of the chromatin substance possesses distinctive characters. Are 

 these the consequence of separate unity, or is there some specific 

 nature of the material ? The history of the hexad multiple answers 

 this question, for although joined to another element the same 

 characteristic behaviour occurs." Summing up the evidence, it 

 may be said that in certain cases persistent individuality is demon- 

 strable directly ; in other cases the reticulum appears to be defin- 

 itely organized with respect to the mitotic complex ; while in 

 mitosis generally the facts in all cases coincide with those which 

 would be anticipated from the assumption that the loss of chromo- 

 somal individuality during interkinesis is only apparent. Further- 

 more, the persistent individuality of chromosomes is a sound 

 working hypothesis ; it permits of verifiable predictions which could 

 not be legitimately inferred without its aid — as, for instance, the 

 existence of octads and hexads in the heterotype complex of forms 

 with multiple chromosomes (see McClung, op. cit). To accept it as 

 such does not imply a denial of the possibility that some mechan- 

 ism may be discovered which could manoeuvre the chromatin 

 granules so as to produce the effects observed without preserving 

 this continuity and integrity of structure. Since, however, Tick, 

 Delia Valle, Meves, Granata, and its critics generally have failed 

 to reveal such a mechanism, the theory of persistent individuality 

 affords at present the most satisfactory interpretation of the nucleus. 

 The earlier exponents of the doctrine under consideration were 

 content to gather evidence in favour of the reality of synapsis, 

 relegating the means by which the conjugation of homologous 

 chromosomes is effected to a position of secondary importance. 

 The study of partial linkages compels enquiry into the latter 

 ])roblem increasingly to-day. It is not enough to interpret the 

 phenomena of independent segregation and coupling of different 

 pairs in terms of localization in identical or tiistinct bivalents ; 

 some mechanism must be found to account for " crossing over " if 

 tlie cliromosome liypothesis is to develop further, and such a 

 mechanism Prof. Morgan's scliool claims to hnd in the twisting of 

 V)ivalent tlireads. It is not possible to criticize here the issues 

 generally raised by the " chiasmotype " theory ; it is sufficient to 

 say tliat such an attempt to provide an interpretation of partial 



