TISSUE CELLS AND CELL TISSUE 



35 



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 

 longer exist alone. 



The division of the egg-cell 

 and its descendants occurs under 

 peculiar inner conditions, which 

 chieflyconcern the nucleus. Direct 

 nuclear division during cell divi- 

 sion is distinguished from indirect 



/^ 



R 





"^STJ 



-x_^ 





.y 



/ 



E 



•^^&&^ 





F 



I 

 J 



or karyokinetic nuclear division. 



The first and, as it appears, the 

 rarer astrees in essentials with that 

 already figured in the Ammha, (p. 

 12, Fig. 19). The second shows 

 various modifications. The follow- 

 ing course may be taken as typical 

 (Fig. 33, A-E). 



[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 

 the constituents of the fibrous 

 net-work ; — and chromatin, 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 chromatin of the nucleus arranges itself as a 

 tangle of fibres (Fig. 33, B). 



2. The nuclear membrane becomes indistinct ; the tangled chromatin 

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



Pig. H.—A-E, Consecutive stages of ceU- 

 aivision, wltli indirect division of the nucleus 

 (diagi'ammatic). 



