THE ANIMAL CELL 27 



nuclei separated from cytoplasm are incapable of continuing 

 their functions. We are at least safe in saying that these three 

 bodies, the centrosome, the nucleus, and the cytoplasm act as 

 intracellular stimuli upon each other, and that all are important 

 in the work of the highly-developed cell. During nuclear 

 division there is probably increased interchange between the 

 cytoplasm and nucleoplasm. The breaking down of the nuclear 

 membrane facilitates this. 



43. Exercises for Library and Laboratory. — It is very helpful for the student 

 actually to study preparations of properly stained cells showing the principal 

 structures; also some of the stages of cell division (see Appendix: Laboratory 

 Suggestions). If no earlier studies have been made of cells and cell division, the 

 growing root tip of Tradescantia, or similar plant, is most satisfactory for gaining 

 first impressions of structure, shape, variation, arrangement, differentiation, and 

 stages of division in cells. 



What are chromosomes? In what respects and to what extent do nuclei differ? 

 What is meant by the "cell-doctrine"? Give an outline of its history. What are 

 mitochondria? Compare the various series of figures in your library illustrating 

 the stages of cell division. 



44. Summary. 



1. The cell may be considered as the unit of structure, and 

 is to be defined as a "nucleated mass of protoplasm with or 

 without a cell wall." 



2. The cell may also be considered the unit of function, in 

 the sense that it embodies all vital functions in epitome. 



3. The structure of the typical cell may be outlined as 

 follows : 



(o) Cell body 



Cytoplasm — living. 

 Cytolymph — non-living, fluid. 

 Metaplasm — non-living, solid. 



(6) Nucleus: 



Nucleoplasm — living. 

 Chromatin. 

 Achromatin. 

 Nucleolymph — non-living, fluid. 

 Metaplasm — non-living, solid. 

 [Protoplasm = Cytoplasm + nucleoplasm.] 



