THE ANIMAL CELL. 25 



41. Functions of the Nucleus and Centrosomes. While we can fol- 

 low some of the externals of the various cell activities, the manner of 

 their occurrence and their causes are in the greatest obscurity. We are 

 not able to say just what part is performed by the different structures 

 involved. It is hazardous to say that one structure is more important 

 than another; yet it seeems to be proven that the nucleus is quite essential 

 in cells which possess nuclei, for the proper performance of even the ordi- 

 nary nutritive functions. Some of the unicellular animals may be* artifi- 

 cially mutilated in such a way that the lost parts may be regenerated and 

 the normal form restored. A relatively small piece of the Protozoan, 

 Stentor, for example, can reproduce the whole, if a portion of the nucleus 

 be present. A much larger piece without nuclear material is wholly un- 

 able to regenerate lost parts, and even seems unable to control or exer- 

 cise the ordinary assimilative functions. The phenomena of indirect cell 

 division show that activity on the part of the centrosomes and nucleus 

 precedes that of the cytoplasm. Experiments also show that the division 

 of the cytoplasm may be checked or interrupted by external influences 

 without interfering with the division of the nucleus. On the other hand 

 nuclei separated from cytoplasm are incapable of continuing their func- 

 tions. We are at least safe in saying that these three bodies, the centre- 

 some, the nucleus, and the cytoplasm act as intracellular stimuli upon each 

 other, and that all are important in the work of 'the cell. 



42. Exercises for Library and Laboratory. The teacher should 

 secure preparations of properly stained cells showing the principal struc- 

 tures; also if possible some of the stages of cell division (see Appendix; 

 suggestions to teachers). 



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. Compare, the various series of figures in your library illus- 

 trating the stages of cell -division. 



43. Summary. 



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

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

 without a cell membrane." 



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 fol- 

 lows: 



(a) Cell body 



Cytoplasm living. 

 Cytolymph non-living, fluid. 

 Metaplasm non-living, solid. 



