1919] Kofoid-SwiZjj: Trkhonympha campanula 75 



so formidable as would seem at first glance. A single chromosome 

 is evidently composed of a ground substance, framework, or thread 

 of linin in which the chromomeres are imbedded and which gives 

 such great consistency to the entire structure. This does not stain 

 and its preseiiee is difficult to demonstrate satisfactorily except as it 

 is outlined by the stained chromatin. 



At the time of division of the chromosomes in Trichonijiiipha 

 (fig. D, 2; pi. 7, fig. 25), a physical continuity may still be retained 

 between the two threads thus fonned, by the incomplete division of 

 this framework. This invisible link, whose existence is suggested by 

 the beliavior of the ends of the chromosomes, would serve to keep one 

 end of each chromosome near the corresponding end of its fellow, and 

 would explain the apparent pairing of chromosomes in these stages 

 (pi. 9, figs. 46—19). These stages, however, have not been observed 

 in the living cell and it is possible that the spreading apart of such 

 pairs may be due to the manipulations of the operator in making 

 smears in the preparation of the material. It is conceivable, however, 

 that considerable separation or strain may occur in the chromosomes 

 of the normal cell without reaching the point of complete separation. 



Given this phj'sical continuity the drawing together of the two 

 threads and their condensation into two shorter parallel tlireads 

 joined at the apex, becomes a simple matter. Such precocious split- 

 ting and separation and their siibsequent union before going on the 

 spindle, have been found to occur in the chromosomes of the tricho- 

 monad flagellates (Kofoid and Swezy, 1915) and in Giardia (Boeek, 

 1917), a procedure which would seem to support an explanation simi- 

 lar to that given above. In view of our present knowledge of cytology, 

 the alternative explanation would be that we have here a synapsis of 

 chromosomes occurring in the ordinary vegetative division cycles, 

 since it is highly improbable, even were these flagellates found to be 

 sexually difi'erentiated, that all the division cycles found within a 

 single j-ear, would be only those of gametes and not the ordinarj' 

 trophozoite division, where such a reduction in the number of the 

 chromosomes would not be expected to occur. Tlie possibility that 

 the behavior of pregametic chi'omo.somes in the Metazoa is a special- 

 ization of a more widely prevalent phase of mitosis in the Protozoa 

 is not precluded. 



Evidences for the precocious splitting of the chromosomes immedi- 

 ately following the end of a division period, have been carefully 

 searched for, but thus far have not been found. At what period in 



