12 THE STRUCTURE OF LIVING THINGS, 



ences and the whole organ is very nearly homogeneous In this 

 case the tissues are ^differentiated, though potentml y capable 

 of differentiation. In the same way, the tissues of the embry- 



FIG. 4. Cross-section through dead wood-like cells from the underground stem of a 

 fern (Pferi* aqu\\\na}. The walls are uncommonly thick and the protoplasm has 

 disappeared. The channels shown served in life to keep the cells in vital con- 

 nection, (x -t50.) 



onic human hand are imperfectly differentiated, and at a very 

 early stage are undifferentiated. 



Tissues composed of Cells. Finally, microscopical examina- 

 tion shows every tissue to be composed of minute parts known 

 as cells, which are nearly or quite similar to one another through- 

 out the whole tissue, and form the ultimate units into which the 

 tissues and organs, and hence the whole organism, become more 

 or less perfectly divided, somewhat as a nation is divided into 

 states and these into counties and townships. 



