ENVIRONMENT ON ARCELLA 



439 



mens were normal and since no specimens of this sort have ap- 

 peared among the thousands reared in the laboratory, it seems 

 safe to conclude that some environmental condition is respon- 

 sible for this peculiarity and that as soon as the controlling 

 factor is removed, the normal characteristics are regained. 



SUMMARY AND CONCLUSIONS 



a. When specimens of Arcella dentata are underfed, the inter- 

 val between successive divisions increases from an average 

 period of 2.50 days to a period of about 4 days; the shell decreases 

 in diameter on the average 2.68 units of 4.3 m each, and the spine 



Fig. 7 Arcella polypora. Camera-lucida sketches (X 207). A = dorsal view 

 of 'wild' specimen with oval shell and mouth. C = side view of same showing 

 bent condition of the shell. B = dorsal view of first offspring of A under labora- 

 tory conditions. D = side view of B. 



number also decreases slightly. The offspring of underfed par- 

 ents produce progeny normal in size and spine number when 

 given an abundance of food, and parents that have been underfed 

 likewise give rise to normal offspring under similar conditions. 

 Wild specimens are often poor in cytoplasmic content, but their 

 offspring are frequently smaller than the parents when reared 

 under laboratory conditions. 



h. Arcella dentata will grow and reproduce in a medium con- 

 taining one drop of sodium silicate to 100 cc. of water. The fis- 

 sion rate decreases, the average interval between fissions increas- 

 ing from 2.50 to 4 days. The size of specimens produced while 

 the parents are in the sodium silicate solution is less than that 

 of progeny formed under normal conditions. The most con- 



