178 PROTOZOA 



A cvtological feature of taxonomic importance is the number 

 of mieronuclei. As mentioned earlier, the number is supposed to 

 be two in P. aiirelia and two to seven, but usually four, in P. mul- 

 timicronucleatum. Because we include both here as P. aurelia, 

 varietal differences in this respect might be expected. 2 A serious 

 difficulty, however, is that much variation in micronuclear num- 

 ber can occur in a single strain, as has been repeatedly observed 

 by a number of investigators. Some of this variation seems to be 

 correlated with the age of the clone under observation. Observa- 

 tions of number of mieronuclei in clones of unknown age are in- 

 secure grounds for identification. Thus, the clones of large 

 paramecia which we have thus far obtained had mostly two or 

 no mieronuclei; but further study, as set forth below, showed 

 clearly that young clones of the same variety normally have four 

 mieronuclei. Experience of this sort leads to the conclusion that 

 micronuclear number in this material becomes a reliable diagnos- 

 tic trait only when used for young clones and only then when a 

 considerable sample of clones is available to indicate the usual 

 condition. 



The young stage, which is in my experience most critical, is 

 the stage shortly before the first fission after fertilization. At this 

 time there are present the differentiated mieronuclei and macro- 

 nuclear anlagen that develop from the products of the first few 

 divisions of the syncaryon. In varieties 13 and 15 the normal 

 condition is to have four mieronuclei and four macronuclear an- 

 lagen. Giese does not give the number for variety 16, but from 

 other information he provides I conclude that this variety will 

 also be found normally to develop four nuclei of each sort. 3 All 

 other varieties normally form two nuclei of each sort. The persist- 

 ent qualification with the word 'normally" is necessary, for vari- 

 ations occur in all varieties. But the normal condition is found 

 in the overwhelming majority of animals at this stage. 



Other cvtological and anatomical features are not as yet suf- 

 ficiently known to be discussed. Among these, the greatest need 

 is for stud) of the silver line system and chromosome number 

 and morphology. The former is under investigation by Powelson 



