No. 2 .] COMPARATIVE CYTOLOGICAL STUDIES. 517 
verschiedene Modificationsstufen des Paranucleins . . . darstel- 
len.” Hessling (’54) found that in the ovum of Unio the smaller 
paranucleolus is divided off from the larger nucleolus proper. 
Hacker, in his last paper on the subject (’95), considers that the 
paranucleoli are of later formation than the nucleolus proper. 
Now in many of those cases where a paranucleolus and a 
nucleolus have been described lying in contact with one another 
it is very probable that the vacuolar portion of the vacuole has 
been described as a paranucleolus. I have no doubt that many 
of the earlier observers, who studied the nucleolus mainly in the 
living egg, have been thus misled, since only sections of nucleoli 
can show the true nature of the nucleolus. Thus Lonnberg, in 
speaking of “helle Kugeln ” in the germinal spot of Mytilus , 
says : “ Es ist schwer zu entscheiden, ob es sich hier nur um 
Vacuolen handelt”; and any one studying the unsectioned nucle¬ 
olus of Montagna would be misled into supposing that here two 
nucleoli of different consistency are apposed to one another. 
Accordingly, we must be very careful in treating as facts some 
of the observations of the earlier workers, which were made 
upon unstained and unsectioned material. 
But there are undoubtedly many cases in which two kinds of 
nucleoli do occur, 1 and this is especially so in germinal vesicles. 
The nucleolus and the paranucleolus may be in contact with 
one another, may be always separated, may at first be in con¬ 
tact and later become separated, or finally may be at first 
separated and later come into mutual contact. Are these para¬ 
nucleoli derived from the nucleolus proper, or have they a 
distinct origin ? In the ovum of Polydora the paranucleoli 
appear towards the close of the maturation period, and then are 
always in contact with the outer surface of the nucleolus proper. 
1 Cf. the reviews of the following papers: Floderus (’96), Hermann (’89a, b, 
’97), Vejdovsky (’95a), Flemming (’74, ’82), Hacker (’93a), Kultschitzky (’88), 
Lukjanow (’87b), Brauer (’91, ’92), Nussbaum (’87), Rein (’83), Henking (’87), 
Van Beneden (’80), Leydig (’55a, ’50), Stauffacher (’93), Stepanoff (’65), Giard 
(’81), Mark (’77, ’81), Lonnberg (’92), Stuhlmann (’86), List (’96), Van Bemme- 
lin (’83), Platner (’86), Clapar£de (’69), Hessling (’54), Ruckert (’94), Bouin (’97), 
Vom Rath (’95b), Moore (’95), Weismann and Ishikawa (’89), Fol (’89), Lacaze- 
Duthiers (’57), Fauvel (’97), Held (’95), Michel (’96), Steinhaus (’88), Metzner 
(’94), Braem (’97), Siebold (’39), Reinhard (’82), Kraepelin (’92), Davenport 
(91). 
