INTRODUCTION 7 



ing' nucleus a network of fibers may be observed 

 similar to the spongioplasm in the cytoplasm ; 

 these consist of a substance named linin because it 

 usually occurs in threads (Fig. 1, /). Distributed 

 along the linin fibers are granules of a substance which 

 stains deeply with certain dyes, and for this reason is 

 known as chromatin (ch). These chromatin gran- 

 ules may unite to form larger spherical masses, the 

 karyosomes or chromatin-nucleoli (&), and during 

 mitotic nuclear division constitute the chromosomes 

 (Fig. 3, C). In many cells one or more bodies 

 resembling the karyosomes somewhat, but differing 

 from them chemically and physiologically, are pres- 

 ent; these are the true nucleoli or plasmosomes 

 (Fig. 1, pi). Embedded in the cytoplasm near 

 the nucleus may often be seen a granular body, the 

 centrosome (c), which is thought to be of great 

 importance during mitotic cell division. The pro- 

 toplasm surrounding the centrosome is usually a 

 differentiated zone, the attraction-sphere (as), con- 

 sisting of archoplasm. The chromatin which may be 

 seen in the cytoplasm of certain cells is as a rule 

 in the form of granules called chromidia (cr). Cer- 

 tain other cytoplasmic inclusions that have attracted 

 considerable attention within the past fifteen years 

 exist as granules, chains, or threads, and are known as 

 mitochondria, chondriosomes, plastosomes, etc. (m). 

 Various sorts of plastids (p), such as chloroplastids 

 and amyloplastids, may be present, besides a varying 

 number of solid or liquid substances, collectively 

 designated as metaplasm (me) or paraplasm, which 



