i;< s EDITH I'INNKN . 



the nature of its morphological relation to the other 

 All such elements of which we have microscopical proof, belong 

 to t\vo classes, tin- accessory or unpaired chromosomes and tin- 

 paired hetefochromosomes or idiochromosomes. This does not 

 take into account those cases like Ciilex and Tlien/xililin where no 

 differential element is visible in the sex cells. In both of these 

 classes the germ cells of one sex hick a definite chromatin mass 

 which is present in the other. The single or unequal pair of 

 chromosomes of the one is replaced in the other by an equal pair 

 both members of which correspond in size respectively to the 

 single clement or to the larger element of the n/ie//iil pair. In 

 Hih()ono'c the search for evidence on this point revealed no pair 

 of chromosomes equal to or exceeding the hook in si/e. This led 

 to the conclusion that a corresponding pair of hooks does not 

 exist. It was then thought that if such a pair was present its 

 members were probably ordinary rods. No rods equal in size 

 to the combined lengths of the two arms were present in any 

 of the eggs. The question as to whether the hook was a multiple 

 chromosome was considered. This probability which I will dis- 

 cuss later, suggested that perhaps the differential sex element 

 consisted of only one arm of the hook. No rods equal in si/e 

 to the long arm of the hook were discovered in any of the eggs. 

 If such rods were present they might escape detection in main- 

 instances but it is improbable that they should be so consistently 

 over-looked in all of the eggs. In Figs. 2a and 2b an attempt 

 was made to draw all of the chromosomes from one spindle. 

 The hook and V's are plainly visible. All of the elements shown 

 could be separated by careful focusing but no rod longer than 

 the arms of the large V's was observed. It appears highly 

 probable that the long arm of the hook is unmatcd in somatic 

 cells containing the hook and that no corresponding pair of Ion- 

 rods is present in cells which contain no hook. It would be im- 

 possible from this material to determine \\hether or not the short 

 arm of the hook was mated. Main short rods approximating 

 it in length occur but the\ could not be studied mdi\ idnally. 

 I have failed to determine ihc number ot chromosome's in 

 these eggs. Figs. i8a, i8b and is, gi\e some of the best polar 

 views of daughter plates. Fig. is,/ shows thirtx -three bodie- 



