Effects of Desiccation on the Rotifer 259 



nature, this takes the form of a conjugation with another individ- 

 ual, but as Maupas, Calkins, and others have shown, other stimuli 

 may serve the same purpose. After division has ceased it may 

 again be started by a change in food or by various chemicals. 

 Perhaps the list of stimuli that produce this effect is larger than we 

 now suspect. Many examples might be given of the effect of 

 external stimuli in causing growth to proceed beyond its usual 

 limits. The production of plant galls by the stings of insects is 

 one of the best known cases. Possibly the formation of certain 

 pathological growths in animals is due to similar causes. An- 

 other example is furnished by the regenerative processes that occur 

 in both plants and animals after injuries, in which resting cells 

 are stimulated into activity. 



In the case of the germ cells, which are potentially immortal 

 the effect of outside stimuli is also clearly seen. In the life of 

 nearly every egg cell, after a period of active growth and division 

 comes a stage in which further development is impossible without 

 some outside stimulus. Usually this stimulus is supplied by the 

 entrance of the sperm. The process of fertihzation has for its 

 purpose two objects, the one being the introduction of new hered- 

 itary qualities and the other the stimulation of the resting proto- 

 plasm of the egg to develop. That these two processes are dis- 

 tinct is shown by the fact that under certain conditions develop- 

 ment may be made to occur in the absence of the sperm. Cases 

 of artificial parthenogenesis have been reported in many groups 

 of animals in which fertilization is the rule in nature. The echino- 

 derms, molluscs, annelids, and insects all furnish examples, and 

 even in some of the vertebrates the early segmentation stages have 

 been obtained by means of appropriate stimuli. Artificial parthe- 

 nogenesis may be brought about in a variety of ways. Heat, che m- 

 icals, and hypertonic salt solutions have all been used with success. 

 Furthermore, as Loeb has shown, a combination of two of these 

 methods may be more effective than one alone. Experiment 

 will doubtless show new methods and new combinations of old 

 methods to secure the same result. 



Is it not possible that in the case of Philodina the process of dry- 

 ing may furnish a stimulus comparable to those just mentioned, 



