202 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
crayfish (fig. 33, p. 182); but I have not been able to find 
distinct evidence of it elsewhere in this animal; and 
although the process has now been proved to take place 
in all the divisions of the animal kingdom, it would seem 
that nuclei may, and largely do, undergo division, without 
becoming converted into spindles. 
The most cursory examination of any of the higher 
plants shows that the vegetable, like the animal body, 
is made up of various kinds of tissues, such as pith, 
woody fibre, spiral vessels, ducts, and so on. But even 
the most modified forms of vegetable tissue depart so 
little from the type of the simple cell, that the reduction 
of them all to that common type is suggested still more 
strongly than in the case of the animal fabric. And 
thus the nucleated cell appears to be the morphological 
unit of the plant no less than of the animal. Moreover, 
recent inquiry has shown that in the course of the 
multiplication of vegetable cells by division, the nuclear 
spindles may appear and run through all their remark- 
able changes by stages precisely similar to those which 
occur in animals. 
The question of the universal presence of nuclei in 
cells may be left open in the case of Plants, as in that 
of Animals; but, speaking generally, it may justly be 
affirmed that the nucleated cell is the morphological 
foundation of both divisions of the living world; and 
the great generalisation of Schleiden and Schwann, 
that there is a fundamental agreement in structure and 
