200 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



from tlie rest, and constitutes tlie nucleus. What part 

 the nucleus plays in relation to the functions, or vital 

 activities, of the cell is as j'et unknown ; but that it is 

 the seat of operations of a different character from those 

 which go on in the body of the cell is clear enough. 

 For, .as we have seen, however diiferent the several 

 tissues may be, the nuclei which they contain are very 

 much alike ; whence it follows, that if all these tissues 

 were primitively composed of simple nucleated cells, it 

 must be the bodies of the cells which have undergone 

 metamorphosis, while the nuclei have remamed rela- 

 tively unchanged. 



On the other hand, when cells multiply, as they do 

 in all growing parts, by the division of one cell into two, 

 the signs of the process of internal change which ends 

 in fission are aj^parent in the nucleus before they are 

 manifest in the body of the cell; and, commonly, the 

 division of the former precedes that of the latter. Thus 

 a single cell body may possess two nuclei, and may be- 

 come divided into two cells b}'^ the subsequent aggrega- 

 tion of the two moieties of its protoplasmic substance 

 round each of them, as a centre. 



In some cases, very singular structural changes take 

 place in the nuclei in the course of the process of cell- 

 division. The granular or fibrillar contents of the 

 nucleus, the wall of which becomes less distinct, arrange 

 themselves in the form of a spindle or double cone, 

 formed of extremely delicate filaments ; and in the plane 



