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.y 





FIG. 7. Adult epithelium. Epidermis of Lumbricus venetra. jchl. z, mucous 

 cells; Cu, cuticula; J.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 



