ORGANIZATIONAL UNITS 27 



metaplasmic in nature and others which are ordinarily con- 

 sidered active elements of the cytoplasm. Specialized living 

 cytoplasmic bodies, known as PLASTIDS, are sometimes also 

 present. Finally, within the cytoplasm in the vicinity of the 

 nucleus, there is frequently visible a differentiated area con- 

 taining a CENTROSOME, an important cell organ which is es- 

 pecially active during cell reproduction. (Fig. 8.) 



The cytoplasm, since it forms the general groundwork, is 

 that part of the cell which comes most closely into relations 

 with the environment, and accordingly near the surface it is 

 frequently modified somewhat in texture and consistency so 

 that an outer region, or ECTOPLASM, may be distinguished 

 from an inner, or ENDOPLASM. Again, the cell may form 

 about itself a definite membrane or a heavy cell wall. Nearly 

 all gradations exist between highly differentiated cytoplasm 

 (ectoplasm) and definite membranes and cell walls. The 

 ectoplasm is certainly a part of the living protoplasm, while 

 certain types of membranes and cell walls must be regarded 

 as non-living, though in many cases they are direct trans- 

 formations of the living materials which grow and play an 

 important, indeed a crucial, part in controlling directly or 

 indirectly the flow of matter and energy to and from the cell 

 and its surroundings. 



2. Nucleus 



Within the cytoplasmic mass there is a restricted area 

 of clearly differentiated material, which typically has a 

 rounded form, bounded by a membrane, so that it appears 

 as a definite body of protoplasm called the nucleus. The 

 structural basis of the nucleus appears to be essentially 

 similar to that of the cytoplasm the so-called LININ mesh- 

 work and KARYOLYMPH being comparable respectively to the 

 granular meshwork and hyaline ground substance of the 



