TISSUE CELLS AND CELL TISSUE 



35 



mmm 



ants produced by repeated fission of the fertilised egg -cell remain 

 bound together in space. Similar cases were found among the 

 Protozoa ; we called them cases of colony formation. While, however, 

 there, all the cells of the colony remained alike and each maintained 

 itself quite like a Protozoan individual, the cell communities of the 

 Metazoa by dividing among the 

 individual cells the various duties 

 of life, so that some cells are ex- 

 clusively adapted for the per- 

 formance of one function, some 

 for the performance of another, 

 raise themselves into stable and 

 well-ordered states, the citizens 

 of which (the cells) are dependent 

 upon one another and can no ^p 

 longer exist alone. 



The division of the egg-cell r^ 

 and its descendants occurs under 

 peculiar inner conditions, which 

 chiefly concern the nucleus. Direct 

 nuclear division during cell divi- 

 sion is distinguished from indirect / 

 or karyokinetie nuclear division. / 

 The first and, as it appears, the I 

 rarer agrees in essentials with that w ; 

 already figured in the Amceba (p. 

 12, Fig. 19). The second shows 

 various modifications. The follow- 

 ing course may be taken as typical (" 

 (Fig. 33, A-H). {: 



[Among the constituents of \ 

 the cell nucleus are to be dis- / 

 tinguished the aehromatin, 



that part which does not stain at 

 all or only very slightly when 

 treated with colouring solutions, 

 viz. the nuclear fluid, and a part of divi sion, with indirect division of the nucleus 



,1 ... ; ,1 T.I (diagram! 



WE* 



Fig. 33. A-H, Consecutive stages of cell- 



,-, ... f . ,! n-i (diagrammatic). 



the constituents of the fibrous 



net-work ; and ehromatin, which freely imbibes colouring matter, viz. 



the nucleoli and other granules of the fibrous network.] 



1. At the beginning of cell division, there appear near the 

 nucleus two opposite attraction centres, round which the portions of 

 protoplasm group themselves in a radiate manner (formation of the 

 amphiasters). The ehromatin of the nucleus arranges itself as a 

 tangle of fibres (Fig. 33, B). 



2. The nuclear membrane becomes indistinct ; the tangled ehromatin 

 falls into several loops (Fig. 33, C). 



