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 ^g%, 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 Montagiia 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,^ 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), Plainer ('86), Claparede ('69), Hessling ('54), Riickert ('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). 



