T. M. SONNEBORN 191 



An immature period appears to be totally lacking in varieties 

 4, 8, 10, and 14, and perhaps in some others. On the other hand, 

 some varieties are not uniform in this respect. Some strains may 

 totally lack an immature period and others may have a short 

 one, its duration varying from strain to strain. Thus, Siegel (un- 

 published) finds that the period of immaturity in variety 1 lasts 

 about four to five days in most strains, although in some strains 

 it may last eight or nine days. I was unable to find any immature 

 period in strain R of variety 1. Comparable variations among 

 strains seem also to occur in varieties 2 and 3. Translated into 

 fissions, these longest periods of immaturity represent about 35 

 or more successive fissions. Much longer immature periods occur 

 in varieties 15 and 16. Giese (1957) reports that some strains 

 of variety 16 did not begin to conjugate until they had been in 

 the laboratory for 6 months (fission rate during this period not 

 stated). In variety 15, I have thus far studied only a relatively 

 small number of crosses, but among them considerable variation 

 in the extent of the immature period was noted even among dif- 

 ferent caryonides of the same exconjugant. In some, it extended 

 for only 8 to 12 days, in others for a little more than two months. 

 However, these cultures were being grown slowly, and this repre- 

 sents a range of about 20 to 100 fissions or more. Curiously, there 

 were relatively few immature periods of intermediate duration. 

 This variability needs further careful study. 5 There thus appear 

 to be at least three different conditions as to immaturity in 

 different varieties of P. aurelia: some have no period of immatu- 

 rity, some have variable but short periods of immaturity, and 

 some have much longer periods of immaturity, at least in most 

 lines. 



3. Maturity. The onset of maturity is marked by the acquisi- 

 tion of the capacity to mate. When there is a preceding immature 

 period, the capacity to mate develops gradually, as evidenced by 

 mating reactions of increasing intensity, i.e., involving larger 

 proportions of the animals. When a period of immaturity is lack- 

 ing, maximal mating reactions occur as early as adequate tests 

 can be made, certainly by the 8th fission after fertilization. Of 

 course, in order to bring about mating, the food supply must be 



