NUCLEAR DIVISION IN PROTOZOA. 



217 



jority of the Flagellata, and may, perhaps, be regarded as the 

 usual condition of protozoan nuclei. Among some Flagellata 

 the aggregation of chromatin granules about the sphere, al- 

 though permanent throughout resting and active phases, resem- 

 bles the loose aggregation of the division period of Tetramitus 

 in having no nuclear membrane (Chilomonas paramcecium, 

 Trachelomonas lagenella, and T. hispidd). In other cases, how- 

 ever, a nuclear membrane is present either as a faint and 

 extremely delicate outline (Synurd) or as a well-defined mem- 

 brane (Euglena, P hocus, etc.). In the latter forms, accordingly, 



FIG. 2. Tetramitus species. A, individual with scattered chromatin granules and central 

 cytoplasmic sphere ; B and C, the chromatin is aggregated in the vicinity of the sphere ; 

 D and E, stages in the division of the sphere and of the chromatin. Optical sections. 



the nucleus has become a definite structure, consisting of chro- 

 matin granules and a kinetic center or sphere (the " nucleolus 

 centrosome " of Keuten and other authors), and nuclear divi- 

 sion takes place without rupture of the membrane. The chro- 

 matic and achromatic elements of the dividing nucleus are thus 

 brought together permanently. 



A third stage in the differentiation of the chromatin is seen 

 in the secondary union of the chromatin granules to form chro- 

 mosomes. In Noctiluca, as described, the process of chromo- 

 some formation is probably as simple as in any case where 

 longitudinal division of the resultant chromosomes takes place. 

 The union of the granules into larger bodies is found in many 



