THE WHEEL ANIMALCULES (ROTATORIA) 581 



the following: (i) large eggs, without a thick shell, from which 

 females are produced; (2) small eggs, similar to the last in ap- 

 pearance but producing males; (3) eggs which have a thick shell, 

 often armed with spines or projections. These are often spoken 

 of as. "winter eggs" or "resting eggs." They may apparently 

 live a long time under all sorts of unfavorable conditions, devel- 

 oping when favorable conditions are restored. The relation of 

 these different sorts of eggs to the appearance of the males, and to 

 fertilization, has been much discussed and investigated. In Hyda- 

 tina, according to Maupas, and in Asplanchna priodonta, according 

 to Lauterborn, the following is the state of the case. There are 

 two sorts of females, not distinguishable externally, but one pro- 

 ducing large eggs, the others small ones. The large eggs cannot be 

 fertilized, and they always develop into female rotifers. The small 

 eggs, if not fertilized, develop into male rotifers, but they may be 

 fertilized, and if this occurs they become transformed into the 

 "resting eggs," from which there later develop female rotifers. 

 In Hydatina, according to Maupas, fecundation can occur only 

 when the female is young, before any of the eggs develop, and the 

 female so fecundated produces only resting eggs. But in As- 

 planchna and in various other rotifers the same female produces 

 both male eggs and resting eggs, although only the latter are 

 fertilized. According to Mrazek, in Asplanchna herrickii the same 

 female bears at the same time ordinary female eggs, male eggs, and 

 resting eggs. There is apparently much variation in these rela- 

 tions among different rotifers. 



Males and resting eggs are as a rule not found at all times of 

 the year, but appear at certain periods, the resting eggs of 

 course following the appearance of males. In the pelagic Rotifera, 

 Lauterborn has made a study of the periodical appearance of males 

 and of resting eggs. He finds that these rotifers may be divided 

 into three classes: (i) perennial rotifers, which occur in greater 

 or less numbers all the year round; (2) summer species, found 

 only in summer; (3) winter species, found only in winter. In the 

 perennial species parthenogenetic reproduction continues through- 

 out the year; but males appear as a rule only twice a year, in 

 spring and fall. In the summer rotifers, males appear in the fall, 



