308 GARY N. CALKINS 



III. THE NUCLEI IN CONJUGATION 



Conjugation between closely related individuals of Uroleptus 

 mobilis is a periodic phenomenon occurring at certain stages in 

 the life cycle. Up to the present it has never taken place in the 

 isolation culture dishes, but ii;! the 'stock' dishes epidemics at 

 certain periods are inevitable. The conditions under which such 

 epidemics occur will be considered in a subsequent paper on the 

 life history of the organism. 



Epidemics of conjugation are invariably preceded by a char- 

 acteristic massing or agglomeration of individuals. These 

 masses vary in size from one-eighth to one-quarter of an inch in 

 diameter and include thousands of individuals so closely packed 

 together that is it impossible to see through the mass. It is a 

 favorable opportunity for securing material for fixation in bulk 

 of the preconjugation and early conjugation stages. 



If such a mass of agglomerated individuals is transferred to 

 another Syracuse dish containing fresh medium, an epidemic of 

 conjugation will invariably follow, and material in different 

 stages may be obtained during the following three days, as the 

 time of conjugation varies from twenty-eight to thirty-six hours. 



The organisms unite and fuse at the anterior ends. The ex- 

 treme tips of the peristomes are the first to unite, and fantastic 

 shapes of the pair, due to independent movements of the two indi- 

 viduals, are characteristic of this early stage. Later, the organ- 

 isms settle into place and fusion of the peristomial grooves fol- 

 lows, until from two-thirds to three-quarters of the peristomial 

 areas are securely united. The mouth regions are never fused. 

 With this union the possibility of independent movements is 

 limited and a characteristic V-form of the pair results. This 

 and its variations are shown in figures 24 to 26. The various 

 stages of maturation and interchange of nuclei are synchronous 

 in the individuals of a pair. 



A. The macronuclei during conjugation 



The eight macronuclei retain their individuality throughout 

 the entire process of conjugation and finally disappear on the 

 fourth or fifth day after separation. While eight is the typical 



