200 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
from the rest, and constitutes the nucleus. What part 
the nucleus plays in relation to the functions, or vital 
activities, of the cell is as yet 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 different 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 remained 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 apparent 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 by the subsequent agerega- 
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 
