156 A. FRANKLIN SHULL AND SONIA LADOFF 



DISCUSSION 



The more recent investigations upon Hydatina, conducted 

 cliiefly by Whitney and ShuU, left no doubt as to the main prob- 

 lem which those investigations were designed to solve. Exter- 

 nal and internal factors both determine the amount of male- 

 production. But the evidence of the operation of external 

 factors was so abundant that new problems were at once created. 

 All such agents at first discovered, and they were not few in 

 number, had the same effect; they reduced male-production. 

 It became important to discover the methods, or preferably 

 method, by which these very diverse agents produced their 

 common result; for by that means it appeared most likely that 

 the solution of the second new problem would be reached. This 

 latter problem was to discover a method of increasing male- 

 production. Only in this way, it seemed, was it likely that the 

 ultimate aim of these studies, the discovery of the physiological 

 phenomena accompanying changes in the mode of reproduction, 

 would be attained. 



The former question, namely, that regarding the modus oper- 

 andi of various chemical substances in retarding male-production, 

 is still unanswered. The experiments described in the first 

 pages of this paper, on osmotic pressure, acidity, and possible 

 after effects of manure solution, led us to no conclusions. . Fail- 

 ure to solve this problem has made the attack of the second prob- 

 lem, discovery of means of increasing male-production, largely 

 a matter of trial and error. Even by this method some success 

 has been attained; experiments 4, 5, and 6 show that calcium 

 chloride has the desired effect upon certain lines of rotifers. 

 Whitney's fortunate discovery that feeding the rotifers on a 

 green flagellate increased male-production gave us our only 

 clew. We have discussed Whitney's experiments above on 

 pages 137 to 139, and will not repeat here. 



Repeated experiments with practically uniform results have 

 demonstrated, to our satisfaction at least, that part of the in- 

 creased male-production following the use of Chlamydomonas 

 as food, in Whitney's cultures, was due to the oxygen liberated 



