242 EDNA MCNALLY. 



shape, brown and the wall is rather thick, presenting a scalloped 

 appearance. They are a great deal smaller than the normal 

 animals and in a newly formed cyst a light spot, the nucleus can 

 be seen. 



Excystment can be brought about by the addition of fresh 

 water and Oscillatoria. After excystment the animal has the 

 characteristics of N. elegans until it encounters food. A few 

 hours after feeding it again presents the characteristics of N. 



ornata. 



4. CONJUGATION. 



In old cultures low in food supply, in which because of their 

 age and food scarcity, encystment might be expected, conjuga- 

 tion has been encountered. Conjugation has only been observed 

 under such conditions and always occurring after the animals 

 have undergone the change from N. ornata to N. elegans. Thus, 

 these adverse conditions of the culture seem to play the greatest 

 role in conjugation. 



The animals approach each other and fuse end to end. The 

 nuclear stages during conjugation have not yet been worked out, 

 but observations of living specimens as well as prepared materials 

 show that the nucleus is not broken down in the process. Calkins 

 (26) states that in many of the protozoa which have been thought 

 to possess a single nucleus, there are in reality both micro- and 

 macro-nuclei. These two nuclei are so closely associated that 

 the micro-nucleus is not visible until conjugation takes place. 

 I am convinced that this is not the case in Nassula, for during 

 conjugation of protozoa where there are both types of nuclei, 

 the macro-nucleus breaks down and the micro-nucleus takes the 

 active part. In Nassula, as stated above, there is no such 

 process, the conspicuous nucleus, which according to Calkins 

 statement would be the macro-nucleus, functioning as does a 

 micro-nucleus during conjugation. 



Many thanks are due Dr. W. A. Kepner of this Laboratory, 

 for his valuable suggestions and aid in carrying on this work, 

 and Miss Margaret Haase for the drawing. 



SUMMARY. 



i. Nassula ornata and Nassula elegans represent not two 

 species, but two metabolic stages of a single species. 



