No. 2.] COMPARATIVE CYTOLOGJCAL STUDIES. 3S9 



or spherical, and after haematoxylin-eosin, stain a rose color, 

 while the chromatin is blue. At the time of the appearance of 

 the chromosomes, " treten die Nucleolen aus den sich zusam- 

 menordnenden Plasma und lassen sich hier in Form scharf 

 umschriebener, homogener, roth gefarbter Kiigelchen nach- 

 weisen." Usually two nucleoli wander out, at least never more 

 than two were found outside of the nucleus. These two come 

 to lie at opposite poles of the nucleus, occupying the positions 

 of centrosomes ; and when the longitudinal splitting of the 

 chromosomes takes place, each of the nucleoli also divides into 

 two. Karsten believes these nucleoli are identical with the 

 centrosomes of Guignard ; but he does not explain what becomes 

 of the third nucleolus during the division. 



Lauterborn ('93), quoted by Karsten ('93), diatoms : there 

 is a centrosome lying in a concavity of the nucleus; he noticed, 

 further, " beim Beginn der Theilung aber zwischen Kern und 

 Centrosom noch ein anderes Gebilde — , welches im spateren 

 Verlauf der Karyokinese eine sehr bedeutsame Rolle spielt, 

 namlich die Anlage der Centralspindel "; this body must be 

 derived either from the nucleus or the centrosome (I mention 

 it here since it may in the future be found to have some con- 

 nection with a nucleolus). 



Moll ('93) studied karyokinesis in Spirogyra. There are one 

 or two nucleoli, which stain more intensely with gentian violet 

 than any other portion of the nucleus. They may be vacuolar 

 in structure, or contain a skein of chromatin ; they appear 

 homogeneous only when too deeply stained. The skein struc- 

 ture (the skein itself staining as chromatin) is found in resting 

 nuclei, as well as in the prophases of mitosis, and at the same 

 time vacuoles may be present. He assumes that the thread in 

 the nucleolus contains all the chromatin of the resting nucleus, 

 and " that by the nucleolus the chromatin substance for the 

 segments [chromosomes] is furnished"; this chromatin leaves 

 the nucleolus in mitosis, and " it seems as if the chromatic sub- 

 stance were squeezed from the nucleolus by an aperture." After 

 the chromatin skein has left the nucleolus, the latter disappears. 



(Strasburger's paper, '93, was reviewed under the head of 

 zoological literature.) 



