No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 327 



Hacker ('923) studied the early development of Aeqnorea 

 forskalea : in the ripe ^gg there is one spherical or kidney- 

 shaped nucleolus, containing vacuoles. At the time of the 

 first pole body mitosis the nucleolus does not accompany the 

 nucleus, but remains behind in the cell, at the place previously 

 occupied by the nucleus ; and from this time on he applies to 

 it the name " Metanucleolus." It is to be observed in one of 

 the cleavage cells until about the 32-cell stage. " Zur Zeit 

 wenn sich dann in der schwarmenden Blastula die Zellen des 

 hinteren Poles . . . zu differenziren beginnen, kann man in 

 einzelnen von ihnen neben dem chromatischen Fadenknauel 

 kleine nucleolenahnliche Korper beobachten, welche den nicht 

 differenzirten Blastula-Elementen fehlen. Es ware denkbar, 

 dass man es hier mit den Abkommlingen des Metanucleolus zu 

 thun hat, ich vermag aber weder hierriiber, noch tiber das 

 weitere Schicksal dieser Gebilde etwas bestimmtes zu sagen." 

 Hacker assumes that what Metschnikoff ('86) supposed to be 

 the " Sperm-nucleus " in Mitrocoma was in reality a Metanucle- 

 olus ; and also that the " Paracopulationszelle," described by 

 Weismann and Ishikawa ('89) in the winter Qgg of Daphnia, 

 to have been also a nucleolus. 



Hacker in a second paper ('92b) studied the maturation of 

 the ovum of Canthocamptiis . In the smallest eggs the nucleolus 

 is large and contains vacuoles. Later it becomes differentiated 

 into a lighter central portion and a denser peripheral part con- 

 taining small vacuoles. At this stage the nucleolus presents a 

 concavity facing the chromatic spirem. Then " aus dem Kern- 

 korper tritt unter plotzlicher Verkleinerung desselben eine Masse 

 aus, welche vermuthlich dem grossen, bis dahin in den meisten 

 Kernkorpern kugligen Einschluss entspricht." The nucleolus 

 apparently disappears when the first pole spindle is perfected. 



Heidenhain ('92), cells of Salamandra : the nucleoli lie 

 enclosed within the chromatin and linin network ; he was unable 

 to decide whether each nucleolus has a particular chromatin 

 envelope. The nucleolus has no processes, and " nur die ihm 

 auflagernde, von ihm selber stofflich differente Schicht ist mit 

 dem Chromatin- und Lininfadengeriist kontinuierlich verbun- 

 den. . . . Mir ist es wenig wahrscheinlich, dass die Substanz 



