Effects of Desiccation on the Rotifer 257 



an egg in the ovary and this one did not develop. No further egg 

 laying occurred. 



This and similar experiments show that the production of eggs 

 is not due to the better food supply after a period of desiccation; 

 since egg production takes place in the absence of food. Further- 

 more, in the cases where a few individuals were kept under direct 

 observation the formation of eggs was sometimes observed to begin 

 before the microzooids had been produced. That the food supply 

 is not without effect, however, is shown by observations such as the 

 following. Seven rather small rotifers showing no trace of eggs 

 were dried on filter paper for three weeks and then placed in a dish 

 of clean water without any food whatever. The development of 

 eggs started just as before and m three days all except one con- 

 tained partially grown eggs. At this point, however, develop- 

 ment ceased, the eggs becoming no larger. Six days later micro- 

 zooids were added from another culture. The rotifers fed rather 

 freely on them and in three days all contained large eggs. An 

 egg laying period then ensued which did not differ from those 

 already described. This experiment shows that although an abun- 

 dant food supply is not the factor that starts egg production, a suf- 

 ficient supply of food must be present in order that the latter shall 

 continue. This is especially the case in smaller rotifers in which 

 the reserve supply stored in the tissues of the body is not great. 



Another factor that must be considered is the amount of oxygen 

 present in the water. It is conceivable that the failure of old cul- 

 tures to produce eggs is due to an exhaustion of the oxygen supply 

 of the water and that the greater activity after a period of drying 

 is due to a renewal of this supply. To this objection it may be 

 answered that cultures in which Sphaerella is present in abundance 

 and in which, therefore, considerable oxygen must be set free in 

 the presence of the light, behave exactly in the same way as cul- 

 tures in w^hich Sphaerella is altogether absent. Furthermore, the 

 addition of fresh water when the culture is almost dry or even 

 after a very short period of desiccation produces no noticeable 

 effect. This last mentioned fact speaks against the view that a 

 diminished production of eggs in old cultures occurs as the result 

 of the accumulation in too large quantities of injurious waste 



