THE STRUCTURE OF CELLS 



formed. These then take up their final positions, and a new set of 

 interzonal fibres are differentiated between them, and in the 

 equatorial planes of these groups of fibres the cell-walls are laid 

 down. And finally, in other cases the cytoplasm may divide into 

 masses containing either single or several nuclei, and secrete 

 membranes without the intervention of interzonal fibres at all. 



It will have become evident from the foregoing account of the 

 relation between the nucleus and the cytoplasm, that these two 

 principal constituents of 

 a cell retain to a consider- 

 able extent a separate 

 individuality, at any rate 

 in the higher forms. This 

 separate nature only be- 

 comes obscured at periods 

 of division, but even here, 

 as has been seen, the 

 essential boundaries are 

 retained through all the 

 changes connected with 

 fission and redistribution. 

 Thus it is legitimate to 

 regard the cell nucleus 

 as an entity which does 

 not arise de now. The 

 nuclei of successive cell- 

 generations are lineal 

 descendants of an ances- 

 tral nucleus, just as the 

 cells of the present day 

 owe their being to the 

 multiplication of ante- 

 cedent parent cells. 



The nucleus, how- 

 ever, does not stand 

 alone amongst the cell 



constituents as only arising by multiplication by fission of pre- 

 existing structures of a similar character. In the plant-cell the 

 various plastids originate in a similar manner, and there is no more 

 evidence to show that they can be differentiated afresh from the 

 general cytoplasm, than that the latter, by spontaneous generation, 

 can arise de now from its elemental constituents. The same is true 

 for the curious coloured plastids known as Zoochlorella in animals, 

 which possibly represent species of algae imprisoned in the cells of 

 their animal hosts, or perchance, though less probably perhaps, they 

 may be regarded as more akin to the chlorophyll corpuscles of the 



D 



FIG. 13. 



Fegatella cemica, the division of the spore-mother-cell 

 into four cells, showing the change in position of the 

 first formed wall. 



