298 C. ISHIKAWA. 



inclined to think. My figures (1 and 2) show the stages of its with- 

 drawal. 



The central protoplasm, which in ordinary conditions takes a 

 more or less elongated form, then becomes concentrated into a star- 

 shaped mass around the nucleus. Close to the nucleus, part of it 

 differentiates itself from the rest by coarser granulation. It is usually 

 spherical in shape (Figs. 1 and 2), but sometimes elliptical (Fig. 31.). 

 This cytoplasmic mass represents the kinetic centre of division, as will 

 be later shown, and corresponds to the sphère attractive of van Beneden, 

 or Arclioplasma of Boveri. In many cases this archoplasmic mass is 

 formed before the complete disappearance of the peristome, teeth, 

 etc., as is shown in Fig. 31. The relative position of the archoplasm 

 and the nucleus does not seem to be always the same j but both 

 bodies lie in the direction of the Stahorgan, as will be seen in Figs. 1, 

 2, and 31. In Figs. 1 and 31 the archoplasm lies behind the nu- 

 cleus, whereas in Figs. 2 and 3 it lies in front of it. In living 

 animals the nucleus at this stage appears as a transparent vesicle, and 

 the archoplasm is very distinct, owing to coarse granulation. 



The archoplasm now stretches itself in a direction more or less 

 in the vertical plane passing through the Stahorgan, and forms a 

 relatively large spindle (Fig. 3.). It undoubtedly corresponds to the 

 central spindle of Hermann (20, p. 580). This spindle gradually 

 elongates and assumes a position longitudinal to the line of division, 

 and divides the nucleus (Fig. 4.). The central mass becomes at the 

 same time separated into two portions, each concentrating around its 

 archoplasm. A narrow line of cytoplasm now appears also in front 

 of the central mass in the direction of the Stahorgan. The spindle 

 jjarts at the middle, dividing the nucleus into two daughter nuclei. 

 These (the daughter nuclei) at first lie in the line of division, but 

 soon change their positions and come to lie right and left of it 



