S16 A. FRANKLIN SHULL 
SUMMARY 
A progressive decrease in the proportion of male-producers 
with long-continued parthenogenesis occurs in some lines of 
Hydatina, perhaps in all. It is not improbable that differences 
between parthenogenetic lines may thus secondarily arise, which 
are independent of both genotypic constitution and the immediate 
external environment. 
A progressive decrease in the size of family with long-continued 
parthenogenesis occurs in some lines. There is apparently no 
correlation between decrease in size of family (decrease of vigor) 
and decrease in proportion of male-producers. 
The time required by fertilized eggs to hatch varies from a few 
days to many weeks. 
The length of time required for a fertilized egg to hatch is prob- 
ably not correlated with the proportion of male-producers in the 
parthenogenetic line derived from the egg. 
Parthenogenetic lines derived from fertilized eggs that require 
a long time to hatch may be less vigorous (as measured by size 
of family) than those from early hatching eggs. 
Individuals hatching from fertilized eggs are not only all 
females, as previously known, but are all female-producers. 
Whether a female is to be a male-producer or a female-pro- 
ducer is irrevocably decided (so far as manure solution is con- 
cerned) in the growth period of the parthenogenetic egg from 
which the female hatches. 
Sex is determined a generation in advance. 
BIBLIOGRAPHY 
Maupas, E. 1891 Sur la déterminisme de la sexualité chez l’Hydatina senta. 
Comp. Rend. Acad. Sci., Paris, T. 113, pp. 388-390. 
Nusssaum, M. 1897 Die Entstehung des Geschlechtes bei Hydatina senta. 
° Arch. f Mikr. Anat. u. Entw., Bd. 49, pp. 227-308. 
Punnett, R. C. 1906 Sex-determination in Hydatina with some remarks on 
parthenogenesis. Proc. Roy. Soc., B, vol. 78, pp. 223-231. 
