HETEROCHROMOSOMES IN THE GUINEA-PIG. 165 



unequal pair may, at least in some cases, be composed of an 

 equal pair of chromosomes with an unpaired heterochromosome 

 fused with one member of the pair, as McClung ('05) has shown 

 to be the case in Hesperotettix. 



The definite cases of regulation in the number i>f heterochromo- 

 somes in the male-producing eggs of Aphis and Phylloxera; and 

 the case ot Rhabditis ni^roi-enosa (Boveri, '11: Schleip, 'u) where 

 a hermaphrodite with 12 chromosomes produces spermato 

 with 5 and <> respectively, by rejecting <>ne heterochromosome 

 in tin- second maturation divi.-ion, naturally suggest that the 

 in of ih,- impaired h.-tep.clironiosome in the male may be 

 attributed to a -imilar regulatory process occurring somewhere 

 in 'lution ot a hermaphrodite or-anism. and i;ivin^ male 



and female-producing -permato/oa. Such a regulation nii-ht 

 hr-t male- ami hermaphrodites, a not uncommon condi- 

 tion, bin tion "1" the male reproductive organs in the 

 hermaphrodite would iv-ult in the u-ual bisexual condition with 

 ihe . ei|iial iii number-, mile (]; ratio \\ere ch.m.id 

 l>\ an environmenl discriminatii in-i one sex, or 1>\ some 



ilalion in ihe number of male and feinale-producin- -perma- 

 tOZOa Ihe une(|iiall\ paired c..ndiiioii of the lieterochromo- 

 -oine- mulii ea-ily ha\e come aboul b\ In-ion ,,f ih<- heierocliro- 

 mosomea in boii \\iih anmhi-r [air of chromosome- in the 



COUl ' reduciioii in ihe number of chroino-ome- 



characterisdc o| th h i- much more difficult to 



ho\\ the change- necessary i" brin^ about I he condition indicated 

 in the -ecoiid lormula could occur, -inc. \\ . mil- inn either 



tor ihe di-appearance of ihre ( A' chromosome- lea\ in;< oiil\- the 

 one A in the male, or !"[ ihe a|)pearanci- of .in A" chromosome 



I'axne'- "09, '10 figures for several of the Reduviidae 



other complication^. 



Some recenl ob-er\ alimis on other material ha\ e impre ed up- 

 on me the probability that the chroinatin units represent in- si \ 

 and other characters may be very -mall indeed, and that in such 



cases as that of C/< 3 M , whereno heterochromosome 



differentiation of .my kind ha> been detected, the members of ;i 

 pair of chromosomes may differ by a sex unit or some other 

 character unit, and the difference in size come within the probable 

 error- of most careful observation. 



