1906.] 



Sex- determination in Hydatina. 



225 



without food during the supposed critical early stages before fresh Euglena 

 was given them 25 hours later. Nevertheless, no $ 's appeared in subsequent 

 breeding from the culture. 



From all this it must be apparent that neither modifications of tempera- 

 ture nor of nutrition afford a satisfactory explanation of the varying 

 proportion of arrenotokous ? 's which may occur in different cultures of 

 Hydatina. There is, however, another explanation of this phenomenon 

 which avoids the necessity of having recourse to external influences. It is 

 possible that it may be the property of certain ? 's to produce arrenotokous 

 $ 's in a definite ratio, and again the property of others to produce none. 

 To test such a possibility it is obvious that the starting point for each 

 generation must be the individual and not, as in all of Maupas' and many 

 of Nussbaum's experiments, a collection of individuals. 



Methods. — Breeding experiments were started in October, 1904, and, with 

 short breaks, have been carried on until the present time. Each rotifer 

 from which offspring were to be reared was isolated in that form of vessel 

 known as a solid watch-glass, where it lived in about 2 c.c. of water 

 containing Euglena. As the young hatched from its eggs they were similarly 

 isolated by means of a fine pipette. If it was desired to test only their 

 immediate offspring, some three or four of these were often placed together 

 for the better economy of space and food. No ? was recorded as either 

 thelytokous or arrenotokous until her eggs had been proved to hatch into 

 either $ 's or <$ 's. The average temperature of the room, as measured by a 

 maximum and minimum thermometer, was about 16° to 18° C. On occasion 

 it was lowered to 6° C. or raised to 24° C. Hydatina is extremely hardy 

 and the chief difficulty in breeding it lies in the food supply, for it thrives 

 far better on Euglena than on anything else. My supply of this Protozoon 

 was derived from Mr. Thos. Bolton, of Birmingham, and was on the whole 

 regular. In the occasional lean intervals particular experiments came to 

 an enforced end, and my chief care was then to preserve certain strains from 

 extinction. In one instance only was I compelled to have recourse for 

 about 10 days to another food supply, and to make use of the bacteria and 

 other small organisms which occur in a decoction of cabbage leaves. 

 Hydatina can live on this diet, but does not thrive as it does upon Euglena. 

 A further difficulty in connection with the food supply lies in its purifica- 

 tion. The water in which Euglena lives usually contains numbers of 

 Hydatina with their eggs. To remove these by a pipette is a tedious 

 business, and there is always an element of uncertainty present in that a 

 young rotifer or an egg may have been overlooked. "Without doubt some 

 of my earlier experiments are not absolutely trustworthy for this reason, and 



