TISSUE CELLS AND CELL TISSUE 



35 



egg -cell 



remain 



ants produced by repeated fission of the fertilised 

 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 



-^.'^. 



. C 



... 



\ 



'"/ 



. 



\ 



1 



"< 



longer exist alone. 



The division of the egg-cell 

 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 

 rarer agrees in essentials with that 

 already figured in the Ar/ueba (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 division - ^th indirect division of the nucleus 

 ,i ... ,. ,, J>, (diagrammatic). 



the constituents of the fibrous 



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



the nucleoli and other granules of the fibrous netAvork.] 



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). 



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



