WEAK PARTHENOGENETIC RACES OF HYDATINA 



SENTA SUBJECTED TO A VARIED 



ENVIRONMENT. 



D. D. WHITNEY. 



Some years ago Weismann maintained that the unicellular 

 organisms were not subject to natural death but were immortal. 

 No individual died from old age but in reproduction went to 

 form the offspring. Later Maupas made some observations 

 upon pedigreed cultures of certain of the protozoa and found 

 that, although the individuals did not develop senile decay, 

 nevertheless, the race did go gradually into senile decay and 

 died out if no conjugation was allowed. Later Calkins, Wood- 

 ruff, Gregory, and others confirmed Maupas's observations but, 

 in addition, Calkins and Woodruff have found that when the 

 races were very weak and near the point of death they could be 

 artificially stimulated and restored to their former vigor by 

 various substances in the food solution. After which they were 

 able to reproduce for many more generations before becoming 

 weak again and then could be restimulated again. However, 

 there always came a time when nothing would reinvigorate the 

 races and they consequently died out. Recently Woodruff has 

 shown that certain races of paramoecia never become weak pro- 

 vided the environment is more or less varied. 



In recent papers Whitney has shown that the rotifer, Hydatina 

 senta, can be propagated parthenogenetically for several hundred 

 generations but each race gradually becomes weaker and weaker 

 and finally dies out. However, when they are in this weak 

 condition the races may be restored to the normal degree of vigor 

 by cross-breeding. Close-breeding within each race only slightly 

 restores their vigor. 



At the time many experiments were made by changing the 

 environment in order to determine whether any external influence 

 would restore these weak races to their normal vigor as had been 



done ih the weak races of protozoa. 



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