48 NATURAL SCIENCE (July 
occurrence of resting-eggs is not confined to any one season of the 
year, though the misleading term ‘ winter-eggs, or even explicit 
statements of the erroneous view implied by it, are still current in 
text-books. Joliet (10) observed that the males and resting-eggs of 
Melwcerta were much more plentiful in dry than in wet seasons, 
Maupas (19) reached the conclusion that temperature was the deter- 
mining factor in the case of Hydatina, and that the sex of the off- 
spring was determined two generations in advance. He states with 
great confidence the view that it is at the moment when the ovum 
is differentiated in the ovary that the temperature influence deter- 
mines whether it shall develop into a male-producing or a female- 
producing individual. His experiments showed that while the 
proportion of male-producing females hatched from eggs laid at 
a temperature of 14°-15°C. varied from 5-24% of the total, it 
rose to 81-100% when the temperature was increased to 26-28°C. 
Against this, Bergendal (21) and Wesenberg-Lund (33) have recorded 
the occurrence in Greenland and Denmark respectively of different 
species of male rotifers in water at a temperature little above zero, 
while Lauterborn points out that the males of most species are con- 
spicuously absent during the warmest months of the year. Nuss- 
baum (30) has recently subjected Maupas’ results to a detailed 
criticism and considers them inconclusive. Working by similar 
culture-methods, he claims to have shown that nutrition is the 
determining factor. He found that females of Hydatina insuticiently 
fed during the early part of their life afterwards laid only male 
eggs, while well-nourished individuals produced female eggs. This 
is obviously in harmony with other well-established instances of the 
influence of nutrition in the determination of sex. 
Lauterborn (32), studying the seasonal variations of pelagic 
Rotifera in the lakes and ponds of the Upper Rhine district, comes 
to a conclusion on this point differing both from those of Maupas 
and of Nussbaum. He believes that the appearance of males and 
the consequent production of resting-eges, in a word, the sexual 
period, is normally recurrent in the life-cycle of each species, after 
a certain definite number of parthenogenetic generations, and is only 
secondarily modified by external conditions, especially in the case of 
those species which have left the relatively stable environment of 
the large lakes for a precarious existence in pools and puddles. In 
this connection he makes an ingenious suggestion which tends to 
reconcile the conflicting results of Maupas and Nussbaum, while 
depriving them of their universal application. Both these observers 
studied Hydatina senta, a species which has supplied the corpus vile 
for very many investigations since the days of Ehrenberg. Now 
Hydatina is characteristically an inhabitant of small accumulations of 
water, wayside puddles and the like, where it feeds on the Huglenae 
