THE NUCLEI OF UROHEPTUS MOBILIS 299 



C. Sections 



The only sure method of getting good sections of conjugating 

 or dividing forms is to embed and section each individual, or 

 pair, separately, after staining with eosin in absolute alcohol. 

 This, however, is a laborious method, and for purposes of con- 

 firming or supplementing the observations on total preparations, 

 fixing and embedding en masse is adequate. For this purpose I 

 use the following method. Only rich cultures of conjugating or 

 dividing fonns should be used. These should be collected in a 

 pipette with as little water as possible and spurted into a test- 

 tube filled with the killing agent. At the same time a quantity 

 of thick zoogloea from an old hay infusion is fixed with the organ- 

 isms to be sectioned. 



The test-tube is thoroughly shaken in order to entangle the 

 organisms in the zoogloea. After these have settled, the fluid is 

 decanted and the process repeated with the different fluids needed 

 for washing and dehydrating. The material is stained with 

 eosin in absolute alcohol, and the zoogloea, now a compact, 

 rounded mass, is finally embedded and sectioned. This method 

 has been employed for many kinds of minute organisms. 



II. THE NUCLEI IN DIVISION 



In the living material early stages of division are easily recog- 

 nized by the reduced length and the increased diameter of the 

 body. If a good culture, which has not been recently fed, is 

 transferred to fresh culture medium, a rich harvest of dividing 

 forms may be obtained in a few hours, and all stages of division 

 will be found amongst them. 



A. The macronuclei 



The eight resting macronuclei of Uroleptus mobilis, all have 

 the same structure (fig. 1). They are densely granular, with a 

 delicate membrane about them, and with the nuclear cleft (Kern- 

 spalt) characteristic of the hj'^potrichous ciliates. As to the sig- 

 nificance of this nuclear cleft, I shall have something to say in a 



