No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 



o/o 



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 ei7ie7i Spahre auswandern und sich hier in die 

 Aussenkornchen umwandeln. . . . Fiir die Kerne der Korn- 

 chenzellen ist dann allerdings, in Gegensatz zu den iibrigen 

 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 ('STb) 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 Myzostomay that the nucleolus 



