THE STRUCTURE OF CELLS 



evidence before us seems to indicate that just as they are very- 

 diverse in structure and composition, so also they may, and almost 

 certainly do, play very different parts in the general economy of 

 the cytoplasm and nucleus. 



The resting nucleus may, then, be regarded as an organised 

 structure containing a considerable assortment of highly complex 

 and labile substances. But this very lability, itself a condition of 

 the profound and important changes which succeed each other with 

 extraordinary rapidity during the division of a nucleus, is bound up 

 in, or at least is related to, an organisation which directs and 



Attraction-sphere enclosing two centrosomes. 



Nucleus * 



Plasmosome or 

 true nucleolus. 



Chromatin- 



nttwork. 



T 1 "*- ' ' 



Lmin-network. i^'J 



Karyosomc or 

 net-] 



Plastids lying in the 

 cytoplasm. 



Vacuole. 



Lifeless bodies (meta- 

 plasm) suspended in 

 the cytoplasmic reticu- 

 luiu. 



FIG. 7. 

 Diagrammatic representation of the structures present in a typical cell. (After Wilson.) 



determines that sequence of chemical and physical transformations 

 which so strikingly accompany the whole process. Moreover, there 

 is abundant evidence of the existence of a material exchange 

 passing between the nucleus and the cytoplasm which becomes 

 strongly marked at all periods of special cellular activity such, for 

 example, as secretion, regeneration, and the like. 



Nuclear and Cell Division. 



The multiplication of the uninucleate cell is always preceded, 

 save in the lowest protozoa and protophyta, in which the details of 

 the processes are still obscure owing to the absence from them of 



