LIFE-HISTORY OF ROTIFER 231 



when the mother was removed from the sUde and which dis- 

 integrated very soon. If eggs are left in isolation slides in old 

 culture medium, hatching frequently does not take place at all; 

 a cloudy, irregular, zone appears around the egg and disintegra- 

 tion soon follows. 



In February, 1919, immediately after the study was begun, 

 sixteen eggs quite different from the usual type described above were 

 deposited. These eggs were 90 x 150// in size, yellowish brown 

 in color, with a lighter yellow circular area in the center, blending 

 to a dark brown at either end. The shell or egg membrane was 

 very thick. All of these eggs were kept in fresh culture medium 

 for three months, under conditions in which the thin-shelled 

 forms hatched readily. No change in appearance had taken 

 place during this time, so they were separated into two lots; one 

 lot was subjected to a temperature of ± 6°C. for a period of two 

 days, then retuiTied to the temperature of the laboratory; the 

 other lot was kept at 36°C. for an equal length of time and re- 

 turned to the laboratory temperature, but no change followed 

 this period of increase and decrease in temperature. Finally, 

 all the eggs were dried for two wrecks at laboratory temperature 

 and then returned to fresh culture solut on, but as no change 

 could be detected, they were discarded at the end of six months. 

 No eggs of this type have appeared during the later stud}?- of the 

 organism, lasting over a period of thirteen months. 



In many species of rotifers tw^o kinds of eggs are produced, 

 thin-shelled ones during periods favorable for rapid multiplica- 

 tion, and thick-shelled forms at the onset of unfavorable condi- 

 tions; these later are known as the 'winter' or 'resting' form. 

 In Hydatina senta, studied so extensively by Whitney and by 

 Shull, three forms are produced, a large thin-shelled form which 

 develops at once into a female, a small thin-shelled which develops 

 at once into a male, and a thick-shelled form which develops after a 

 period of rest into a female; the first two forms are produced 

 parthenogenetically, the third by a fertilized female. The 

 thick-shelled eggs of Proales are comparable to the 'winter' 

 eggs described above, since they appeared at a time when re- 

 production under normal conditions might be accomplished 



