ZOOLOGY: ZELENY AND FAUST 
93 
For three of the species studied, Musca domestica, Alydus pilosiilus 
and Anasa tristis, there are good published figures of the chromosomes 
in the spermatocyte divisions. From these figures it is possible to make 
a rough estimate of the total chromosomal volume in each of the two 
kinds of cells. The expected ratio between the head-lengths of the 
two kinds of resulting spermatozoa was calculated from these data. 
A close agreement was discovered between the calculated ratios and 
the observed ratios. For Musca domestica the calculated ratio is 
1.00 : 1.08 and the average of the observed ratios 1.00 : 1.07. For 
Alydus pilosulus these ratios are 1.00 : 1.06 and 1.00 : 1.055. For Anasa 
tristis the calculated ratio is 1.00 : 1.11 and the average of the observed 
ratios 1.00 : 1.09 with six of the nine determinations located between 
1.00 : 1.10 and 1.00 : 1.12 
While there undoubtedly is a considerable error in making minute 
measurements like those here described and while there may be doubt 
as to the significance of some of the individual determinations yet the 
data when taken as a whole must be interpreted as showing the very 
general existence of dimorphism in size among the completed sper- 
matozoa. The evidence further makes it very probable that this di- 
morphism is the result of the chromosomal dimorphism present in the 
spermatids. If this conclusion is a correct one the hypothesis that the 
chromosomal dimorphism is related to sex determination may be sub- 
jected to direct test as soon as a method can be devised for separating 
the different sizes of living spermatozoa. 
TABULATION OF DATA ON HEAD-LENGTHS OF SPERMATOZOA 
The observed ratio is the ratio between the two modal values. The 
expected ratio is calculated from the chromosomal difference between 
the two kinds of spermatids. 
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