No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 
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Oder chemischen Umwandlung anheimfallen.” In only one 
astrosphere of only one cell in each of the following cleavage 
generations this process is repeated, and the line of these par¬ 
ticular cells (“ Kornchen-Zellen ”) constitutes the line of devel¬ 
opment of the sexual cells ; but the ectosomes are present in 
these particular cells only during mitosis, and in the resting 
stages are absent, while nucleoli occur in the nuclei ; this proc¬ 
ess was observed from the first through the ninth cleavage 
stages. He concludes that in each generation there is a produc¬ 
tion de novo and a subsequent solution (“Auflosung ”) of the 
ectosomes. The first appearance of the latter coincides in point 
of time approximately with the disappearance of the nucleolar 
substance in the nucleus ; from this and certain other factors 
he concludes : “ So . . . wiirde also der Annahme kaum etwas 
im Wege stehen, dass die zu Beginn der Mitose noch vorhan- 
denen oder neugebildeten Nucleolen aus dem Kernraum in der 
Richtung der einen Spahre auswandern und sich hier in die 
Aussenkornchen umwandeln. ... Fur die Kerne der Korn- 
chenzellen ist dann allerdings, in Gegensatz zu den ubrigen 
Embryonal-Elementen, eine besonders reichliche Produktion 
der Nucleolarsubstanz und demnach eine besonders intensive 
vegetative Thatigkeit [cf. ’95] anzunehmen.” The explanation 
for the arrangement of the ectosomes in only one of the 
astrospheres he finds in the assumption “dass die beiden 
Centrosomen einen verschiedenen (vielleicht einen verschieden 
* kraftigen ’) Einfluss auf das umgebende Plasma, beziehungs- 
weise auf die beweglichen Inhaltskorper desselben ausiiben.” 
Hacker (’ 97 b) finds that in germ cells of both animals and 
plants there is to be noted “das Auftreten eines einzigen, 
vacuolenhaltigen, dunkel tingierbaren “ Hauptnucleolus ” in 
den jiingeren Stadien, das Hinzutreten von blasseren adventiven 
oder “ Neben-Nucleolen ” in einer friiheren oder spateren 
Phase.” Nucleolar substance arises during one or several stages 
of nuclear activity as a by-product of metabolism, possibly also 
as chromatin substance which has become structureless and 
chemically changed; and, finally when the nucleus begins to 
divide, is removed out of the latter. He confirms Wheeler’s 
(’ 96 ) observations on the ovum of ATyzostoma, that the nucleolus 
