No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 
Hacker (’92a) studied the early development of Aequorea 
forskalea: in the ripe egg 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 hierrfiber, noch fiber 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 egg of Daphnia , 
to have been also a nucleolus. 
Hacker in a second paper (’92b) studied the maturation of 
the ovum of Canthocamptus . 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 Lininfadengerfist kontinuierlich verbun- 
den. . . . Mir ist es wenig wahrscheinlich, dass die Substanz 
