92 



CELL-DIVISION 



Still simpler conditions are found in some of the flagellates.^ In 

 CJiilomonas the sphere may still be regarded as intranuclear, since it 

 lies in the middle of an irregular mass of chromatin-granules, though 

 the latter are apparently not enclosed by a membrane. Nuclear 

 division is here accomplished by fission of the sphere and the aggre- 

 gation of the chromatin-granules around the two products. In 

 Tetramitus, finally (Fig. i6), the nucleus is represented by chromatin- 

 granules that are scattered irregularly through the cell and only at 

 the time of division collect about the dividing sphere. 











B C 







^^PP^ ^^rfr "^ fill 



D E F G 



Fig. 41. — Mitosis in the Heliozoa. [SCHAUDINN.] 



A, SphcBrastriim ; vegetative cell showing nucleus, "central granule" (centrosome), and axial 

 rays. B-G. Acanthocystis. B~D. Prophases of mitosis. E. Budding to form swarm-spores. 

 F. Swarm-spores, devoid of centrosomes. G. Swarm-spores preparing for division ; intranuclear 

 origin of centrosome. 



In a second series of forms, represented by Noctiliica (Ishikawa, 

 '94, '98), (Calkins, '98, 2), Paramocba (Schaudinn, '96, i), Actinophrys 

 and Acanthocystis (Schaudinn, '96, 2), and the diatoms (Lauterborn, 

 '96), the sphere lies outside the nucleus in the cytoplasm and the 

 mitosis is closely similar to that observed in most Metazoa. This is 

 most striking in the Heliozoa, where the centrosome persists through 

 the vegetative condition of the cell as the " central granule," to which 

 the axial filaments of the pseudopodia converge. Schaudinn ('96, 2) 

 shows that by the division of this body a typical extranuclear amphi- 

 aster and central spindle are formed (Fig. 41), while the chromatin 



1 Calkins, '98, I, '98, 2. 



