THE NEW CELL DOCTRINE 



gether. When such an epithelium grows the nuclei multiply 

 by division which is in itself a complicated process. The pro- 



FiG. 6. — Mesenchyma of a chicken embryo of the third day of incubation. 

 Every nucleus is surrounded by a thin layer of protoplasm from which run out the 

 strands that form the intercellular network. Cell boundaries are not present. 



toplasm also grows. We have in this case, therefore, a sub- 

 stance which, though living, does not, strictly speaking, con- 



, schl.n 



Fig. 7. — Adult epithelium. Epidermis of Lumbricus venetra. ocM. s, mucous 

 cells; Cm, cuticula; d.z., cylinder cells; m.f, muscle fibers — below the epithelium. The 

 single cells are separated by partition walls from one another. — After M. Heidenhain. 



sist of cells. The second form of tissue is called mesenchyma. 

 In mesenchyma. Fig. 6, one observes nuclei which are found 



