No. 2.] COMPARATIVE CVTOLOGICAL STUDIES. 



Z7. 



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 produ^c- 

 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 . . . wurde 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. ... Fiir die Kerne der Korn- 

 chenzellen ist dann allerdings, in Gegensatz zu den ubrigen 

 Embryonal-Elementen, eine besonders reichliche Produktlon 

 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 ausuben." 



Hacker ('gvb) 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 jungeren 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 Myzostoma, that the nucleolus 



