258 Merkel Henry Jacobs 



products. No matter how large the volume of water added no 

 increase in egg production occurs without a fairly long desiccation. 

 In cases where rotifers were transferred to other dishes and placed 

 in fresh water there seemed to be no noticeable increase in repro- 

 ductive activity. 



Although desiccation is able to cause the formation of eggs at 

 times when they would not otherwise appear it is not always neces- 

 sary for their production. In an experiment made to determine 

 the length of Hfe of Philodina, several individuals, raised from the 

 egg without any desiccation, when about a week old began to 

 lay eggs and continued to do so for several weeks. Furthermore, 

 it must be noted that under more natural conditions than can be 

 obtained in the laboratory, eggs seem to be produced more freely 

 than in the cases mentioned. Their production does not seem to 

 be so clearly related to a period of desiccation. This may be 

 partly due to the greater variety in the conditions met with out of 

 doors in a large stone urn and partly to the more abundant food 

 supply which would permit the effects of a single drying to last 

 for a longer time. However this may be, and whether or not 

 desiccation is the only factor concerned in the production of eggs 

 the experiments described leave little room for doubt that it is 

 a factor and under certain conditions a most important one. Un- 

 der these conditions the process of desiccation serves as a stim- 

 ulus to cause the production of eggs at times when otherwise they 

 would not be produced. The drying in some way starts into activ- 

 ity the previously resting germ cells and inaugurates a process of 

 growth and division which may continue for a considerable time 

 after the initial stimulus has ceased to act. This fact is of some 

 interest when considered in connection with certain other well 

 known phenomena relating to the development of both plants and 

 animals. 



It is well known that most cells cannot continue to grow and 

 divide indefinitely, without receiving certain stimuli from without. 

 The behavior of some of the Protozoa furnishes a striking exam- 

 ple of this point. Several workers have shown that in ParamcE- 

 cium after a number of generations division becomes slower and 

 finally ceases unless an appropriate stimulus be supplied. In 



