No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 511 



it is not impossible tliat the supposed nucleolinus was in reality 

 a microorganism inclosed in the vacuole of the nucleolus. 

 {Cf. also Flemming's observation on the egg of Ascidia, '97.) 

 Supposed nerve fibrils in the nucleolus have been described by 

 Eimer ('73, '90). 



5. Polarity of the Nucleolus. 



In the gregarine {Gotiospora f) from the intestine of Litieiis 

 gesserensis it is the rule that the vacuoles make their first 

 appearance at that pole of the nucleolus which is nearest to the 

 nuclear membrane. In the germinal spot of Montagua the 

 opposite position of the large excentric vacuole is the rule, 

 though the percentage of cases in which the vacuole has a 

 particular position with regard to the nuclear membrane is 

 less than in the gregarine. On the contrary, in the germinal 

 spots of Piscicola and Rodalia there is no regularity in regard 

 to the position of the vacuoles, and in that of Polydora the 

 vacuoles are, at the time of their first appearance, usually 

 central in position. In the germinal spots of many other 

 Metazoa, where a single large vacuole is present it more 

 usually lies excentrically than centrally, though its position 

 appears to be independent of the proximity of the nuclear 

 membrane ; so that in these cases we can speak of a certain 

 polarity in regard to the position of the vacuole within the 

 nucleolus, and not of a polarity of the axis of the nucleolus 

 in regard to the position of the nuclear membrane. But 

 in the two gregarines examined by me the substance of the 

 nucleolus, or of some of the nucleoli, is differentiated at 

 two poles of the nucleolus, so that the portion of the ground 

 substance at one end stains differently from that of the other 

 end of the nucleolus ; this state apparently does not occur in 

 the nucleoli of metazoan cells. It remains to be solved whether 

 in the gregarines the chromatin or its physiological equivalent 

 is localized at some particular point or pole of the nucleolus, 

 i.e., whether or not such nucleoli should be compared to the 

 nucleoli of the Metazoa. 



