12 



THE STRUCTURE OF LIVING THINGS. 



ences and the whole organ is very nearly homogeneous. In this 

 case the tissues are undifferentiated^ though potentially capal)le 

 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 (Ptei'is aquilina) . 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 450.) 



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 shnilar 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. 



