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 

 Melicerta 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 insufficiently 

 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 

 Eotifera 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-eggs, 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, scnta, 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 Euglenae 



