STUDIES ON CHROMOSOMES 421 



for the production of the sex-limited characters are born by 

 the X-chromosome; and without this assumption they are wholly 

 mysterious. 



Adopting this explanation, the history of certain of these sex- 

 limited characters, as Morgan points out, demands the further 

 assumption that in the female the factors for these characters 

 may to some extent undergo an interchange between the two 

 sex-chromosomes (here two .XT-chromosomes) while in the male 

 such interchange does not take place. Such a difference between 

 the sexes finds a perfectly simple explanation in cases where the 

 X-chromosome of the male has no synaptic mate (F-chromosome). 

 When a F-chromosome is present as may be the case in Droso- 

 phila according to the cytological observations of Stevens ('08) 

 the problem becomes more complicated, but there are some facts 

 that may be significant in this direction. It is well known that in 

 the male the sex-chromosomes commonly retain a compact and 

 rounded form (as 'chromosome-nucleoli') throughout the entire 

 spermatogenesis, and in some cases (Oncopeltus) they conjugate 

 while in this condition, and subsequently disjoin without ever 

 having undergone fusion or even intimate union, such as is so 

 characteristic of the autosomes during the maturation-process. 

 Unfortunately the oogenesis is as yet but imperfectly known; 

 but there is considerable evidence that in some forms the sex- 

 chromosomes exhibit a quite different behavior from that char- 

 acteristic of the spermatogenesis. My own observations ('06) 

 seemed to show that in some of the Hemiptera the sex-chromo- 

 somes do not in the oocyte retain a compact form at the period* of 

 synapsis, or in the early growth-period, and the later observations 

 of Foot and Strobell ('07) show that the same is true for later 

 stages of the germinal vesicle. The work of Morrill ('10) further 

 shows that in the female the sex-chromosomes have already con- 

 jugated before the first oocyte-division. These fact make it 

 probable that in these forms the sex-chromosomes of the female 

 show the same behavior in synapsis and reduction as the auto- 

 somes, and enter into the same intimate union that characterizes 

 the latter. It is quite possible that in these facts we may find an 



