86 Sara White Cull 



gamete, completely fuses with a smaller cell of greater activity, a 

 microgamete. The complete union oftwo cells alongwith differences 

 in size and activity are characters which distinguish the process 

 of fertilization as usually understood, from the process of conju- 

 gation, as seen in forms like Paramecium. Both processes agree 

 in having the same essential feature, the union of the nuclei of the 

 two cells. In the different classes of protozoa all steps may be 

 found from conjugation in a general sense to a process exactly 

 similar to fertilization used in a strict sense. Even the maturation 

 phenomena which play so important a role in the history of 

 metazoan germ cells are represented in some sort by processes 

 which have been observed in a few protozoa. 



In isogamous union, such as that which takes place in Para- 

 mecium caudatum, two individuals of the same size and approxi- 

 mately equal activities unite for a short time, and the ectoplasm 

 around the mouths of the two organisms fuses to form a sort of 

 bridge over which the nuclei pass. During the maturation 

 phases, previous to this nuclear exchange, the micronucleus of 

 each organism gives rise by division to four or more pronuclei. 

 Two of these are destined to be functional and the others, cor- 

 puscles de rebut, as Maupas calls them, disintegrate. One of the 

 two functional pronuclei passes into the other organism where it 

 fuses with the stationary pronucleus of that cell, forming one single 

 reorganization nucleus. From this, by repeated division arise 

 the micronucleus and macronucleus of the rejuvenated protozoan. 

 These organisms then proceed to reproduce by ordinary fission. 



The species used for the experiments described here was Para- 

 mecium caudatum and the material was what is known as "wild." 

 Each conjugating pair was taken up in a fine pipette and put into 

 a hollow slide containing some drops of the culture liquid — hay 

 infusion — ^free from all other protozoa. These slides werethen put 

 into moist chambers. In all cases an examination was made after 

 the isolation of the conjugating pairs to see that they had not been 

 separated in the process of handling, for if this precaution were not 

 taken, one could not be sure of dealing with the results of conjuga- 

 tion. On the day following isolation, when, in most cases, the 

 exconjugants were swimming freely through the water, each one 



