240 EDNA MCXALLY. 



plays a small part in the converting of TV. ornata into the so-called 

 N. elegans. Older cultures of N. ornata change a short while 

 before those in which the water is fresher. This will be seen by 

 referring to Table I. Hence we find that N. ornata and N. 

 elegans are but metabolic phases of a single species based upon 

 nutritional and toxic conditions. 



3. ENCYSTMENT. 



As stated above, after food has been withheld from .V. ornata 

 the animals develop the characteristics of N. elegans. If now, 

 food is again added to these specimens a reversal will occur and 

 the animals will again appear as N. ornata. This may be carried 

 on indefinitely making alternate sequences of the forms so long 

 as food is withheld and added at the proper time and in the proper 

 sequences. However, if no food is added, the animals eventually 

 become much lighter in color than even N. elegans, becoming 

 almost transparent. Then encystment takes place. Thus it 

 happens that they enter the cysts as TV. elegans, N. ornata has 

 not been observed to encyst. 



Age of the culture aids in encystment but is not the basic 

 cause as will be seen from the following specific experiment: 



Four cultures were started as follows: All the Nassulas being 

 taken from the same culture which was a sub-culture of the 

 original pure line. 



Culture No. I. Water from a culture of Nassulas several weeks 

 old was filtered and placed in a watch glass with approximately 

 twenty-five TV. ornata. No Oscillatoria was present. 



Culture No. 2. Fresh spring water, about twenty-five TV. 

 ornata and no Oscillatoria. 



Culture No. j. Water from the same vessel as No. I, about 

 twenty-five TV. ornata and an abundance of Oscillatoria. 



Culture No. 4. Fresh spring water, about twenty-five TV. 

 ornata, and an abundance of Oscillatoria. 



All of the four cultures were kept in square watch glasses 

 and these placed in a larger glass dish so that the external condi- 

 tions were the same. 



The cysts are formed along the sides of the vessel or in the 

 detritus on the bottom of the vessel. They are spheroidal in 



