202 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



crayfish (fig. 33, p. 132) ; 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 w^ould seem 

 that nuclei ma}^ 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 



