270 



THE PROTOZOA 



that of the intra-nuclear division-centre in Euglena (Fig. 142). In 

 Noctiluca the central spindle, which is derived from the extra-nuclear 

 division-centre, takes a similar intra-nuclear position during division 

 of the nucleus, and when the nuclear membrane disappears in the ad- 

 jacent portions of the nucleus, the kinetic substance is again intra- 

 nuclear, although the centres of attraction are outside (Fig. 141, D, E). 

 In Actinosphcerium, both Brauer ('94) and Hertwig ('98) explained the 

 cytoplasmic accumulations (yProtoplasinakegel) as coming from the 

 nucleus, and Hertwig pictured the outer mass and the inner achro- 

 matic substance as connected through openings in the membrane 

 (Fig. 140). Confirmatory evidence is also shown in other Heliozoa. 

 In Actinophrys there is no extra-nuclear division-centre, and Schau- 



--s- 



D 



B C 



Fig. 143. — Mitosis in Tetramitus chilomonas. 

 A. Ordinary form with distributed chromatin (c) and division-centre (s). B. The chromatin' 

 granules are collected prior to division. C. The division-centre has divided. D. Later stage in 

 division ; each daughter-nucleus is surrounded by a group of chromatin granules. 



dinn ('96) interprets the achromatic spindle figure as nuclear in origin 

 (Fig. 130, p. 236). 1 



The flagellate Tetramitus shows an apparently similar division- 

 centre. During the resting phases, the chromatin is distributed 

 throughout the cell, while an indefinite "achromatic mass" appears 

 to be in direct connection with the cytoplasmic reticulum. Imme- 

 diately before division, however, the chromatin granules collect about 

 this body, and then, save for the absence of a membrane, the aggre- 

 gate resembles the nucleus of Euglena. Division takes place as 



1 A significant fact is that in Actinophrys the radiating axial filaments centre in the 

 nucleus, while in other Heliozoa with a "Centralkorn" the radiating axial filaments centre 

 in this extra-nuclear body. This indicates that in Actinophrys the attraction-centre is withim 

 the nuclear substance and presumably in the " achromatic substance." 



