260 



THE PROTOZOA 



division, it elongates to form a dumb-bell-shaped body, the two ends 

 remaining connected until the end of division, by a strand. After 

 division the daughter-centre rounds out and resumes its customary 

 form and position within the group of chromatin granules. 



An essentially similar process was described by Schaudinn ('94) in 

 the division of the rhizopod Amoeba crystalligera, and although he 

 recognized that the intra-nuclear body plays the chief role in division, 

 he regarded it as the nucleolus (Fig. 137). It has since been found 

 in the majority of Mastigophora, in many Rhizopoda, Heliozoa, and in 



Fig. 136. — Mitosis in Euglena. [WILSON after KEUTEN.] 

 A. Preparing for division ; the nucleus contains a " sphere " or division-centre, surrounded by 

 a group of chromosomes. B. Division of the sphere to form an intra-nuclear spindle. C. Later 

 stage. D. The division completed. 



Infusoria. In many cases it loses its distinct outline and becomes 

 more or less indistinct, although reappearing during cell-division as 

 the division-centre. In the dinofiagellate Ceratium liirumiinclla, 

 Lauterborn ('95) described it as indistinct and apparently without the 

 usual function. 



The intra-nuclear division-centre becomes difficult to see, at least 

 in the resting phases, in micronuclei of the Infusoria, in the rhizopod 

 Euglypha, in the heliozoon Actinosphcerium, and in the ciliate Spiro- 

 chona, forms which may be selected as showing the variations in the 

 intra-nuclear division-centre. 



