NERVE ELEMENTS AND THEIR FUNCTIONS. 87 



in the course of embryonic development the number, form, posi- 

 tion and multiplication of cells do not determine the course of 

 differentiation. The cells are not controlling organisms which 

 possess the iniative and directive power in these processes, but 

 are perhaps only mass divisions made necessary by the metabolic 

 relations of the nucleus and cytoplasm. The course of develop- 

 mental differentiation is determined by a more fundamental 

 organization which exists in the whole protoplasm of the organism 

 before its division into cells, and by the interactions between the 

 organism and its environment during development. The cells 

 are rather the plastic material of differentiation. As the processes 

 of differentiation complete themselves, however, cells and groups 

 of cells become the means or organs for the performance of certain 



FIG. 43. A portion of the network about the walls of a small vessel in the 

 palate of the frog. From Prentiss. 



functions. Organs are cell- complexes and the function performed 

 by an organ is the mass result of the functioning of its constituent 

 cells. Thus it may be stated as a general rule that each mass of 

 protoplasm containing a nucleus is essentially a structural and 

 functional unit of the organ. It is the nucleated mass of protoplasm 

 which maintains itself as a structural entity although it may be 

 connected by plasm strands with its neighbors, and which per- 

 forms its specific part of the function of the organ. 



These considerations apply to the nerve cell as well. It has 

 been clearly shown that in many cases nerve cells are in continuity 

 with one another. In the case of some peripheral plexuses (Figs. 



