116 



of the microgamete with the anterior nucleus of the binucleated macro- 

 gamete. The phenomena immediately following conjugation have not yet 

 been fully determined. They are too various for brief description (fig. 5). 



The tailed gametes often, possibly always, arise by differential divi- 

 sion, one product of the division acquiring a tail, the other not. The 

 tailed gamete may itself begin to divide again by longitudinal fission 

 before it has completely separated from its sister. I do not know how 

 many divisions occur after an animal leaves its infection cyst before the 

 gametes arise. The size of the infection cyst and of the animals that 

 hatch from them varies so much that measurements of the minute forms 

 cannot settle this question, and I do not succeed in rearing the isolated 

 animals long enough to answer it. 



The chromosome number is four for the microgametes and the 

 macrogametes of 0. intestinalis (fig. 6 and 7). It is three for the ga- 

 metes of 0. caudata (fig. 4). These chromosomes, as seen in the living 

 animals, show about half as many granules as do the chromosomes of 

 the full grown individuals. I hope by the study of sections to deter- 

 mine when and how this reduction occurs. It seems clearly to take place 

 before the formation of the infection cysts. Neresheimer finds twelve 

 chromosomes in the adult 0. ranarum and twenty-four in the young 

 spring forms before encystment, a result I do not understand. 



Opalina dimidiata. 



The infection cysts contain from two to seven nuclei, generally 

 from three to five, most commonly four. The animals hatch from the 

 cysts apparently with the same number of nuclei they had on entering 

 the cyst. Both longitudinal and transverse division occurs after hatching. 

 Tailed gametes of two sizes are formed as in the binucleated Opa- 

 linae. These I have never seen conjugate with one another. Of the 

 conjugating gametes one was always found to be tailed and the other 

 without a tail and usually very much larger. Normal encystment after 

 conjugation was not found in this species, nor have I seen fusion of the 

 nuclei in the zygote. The number of chromosomes in the gametes or the 

 adults has not yet been determined. It is much less in the gametes, 

 apparently half of the number of the adults. The tailed gametes always 

 have one nucleus. The tailess macrogamete may have one or two. 



At about the time the infection cysts are formed, peculiar pheno- 

 mena are observable in the nuclei of 0. intestinalis, 0. caudata and 0. 

 dimidiata, which I will not attempt to describes in this condensed paper. 

 They seem to be associated with a reduction in the amount of the functio- 

 nal chromatin and somewhat resemble the phenomena attending de- 

 generation of the nuclei in abnormal 0. ohtrigona. The most noticeable 



