No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 511 
it is not impossible that 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 (Gonospora f) from the intestine of Lineus 
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 Montagna 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. 
