HYDATINA SENTA SUBJECTED TO ENVIRONMENT. 329 



sented in former papers. However, the point to be noted in this 

 table is that this race A, which is near the point of dying out 

 from general exhaustion after having lived through more than 

 500 parthenogenetic generations, is still capable of producing a 

 high percentage of male-producing females. 



SUMMARY AND CONCLUSION. 



These results together with those of former papers show that 

 three races of Hydatina senta kept under a constant environment 

 gradually became weaker when allowed to reproduce partheno- 

 genetically for several years. 



Two races, B and A , have finally died out in the 384th and the 

 546th parthenogenetic generations respectively. The other race 

 C has been discontinued in the 443d parthenogenetic generation 

 but also showed a marked lowering of vitality. 



2. A wide variation in the food media including the micro- 

 organisms used as food and also the chemicals in solution did not 

 reinvigorate the weak races. Extracts of beef, various glands of 

 a sheep and cat, the diabasic salt potassium phosphate also were 

 valueless as stimuli. 



3. Close-fertilized eggs of the weak races subjected to periods 

 of rest for a year in water and for eight months in the dried 

 state, to a temperature of --70 C. in water and to temperatures 

 of 191 C. and +100 C. in the dried state, 6-96 hrs., failed 

 to produce any reinvigoration. 



4. The final conclusions are: that parthenogenesis can continue 

 for several hundred generations but results in the gradual 

 weakening and final extinction of the race; variation in the 

 environment including food, chemicals in the food media, and 

 temperature, do not cause a reinvigoration in such weakened 

 races. 



5. Some races show no progressive decrease in the proportion 

 of male-producing females with long-continued parthenogenesis. 



BIOLOGICAL LABORATORY, WESLEYAN UNIVERSITY, 



MlDDLETOWN, CONN., July l8, IQI2. 



