vi MINUTE STRUCTURE OF NUCLEI 63 



matrix traversed by a delicate network. In the epithe- 

 lial cells of animals the protoplasm is bounded exter- 

 nally by a cell-membrane (Fig. 9, A, c. m) of extreme 

 tenuity, in plants by a cell-wall (B, c. w) of cellulose : in 

 amoeboid cells the ectosarc or transparent non-granular 

 portion of the cell consists of clear protoplasm only, the 

 granular endosarc alone possessing the sponge-work. In 

 the majority of full-grown plant cells (Fig. 8, F) and in 

 some animal cells the protoplasm is more or less exten- 

 sively vacuolated, but in the young growing parts as well 

 as in the ordinary cells of animals the foregoing description 

 holds good. It is quite possible that the reticular character 

 of the protoplasm may be merely the optical expression of 

 an extensive but minute vacuolation, or may be due to the 

 presence of innumerable minute granules developed in the 

 protoplasm as products of metabolism. 



The nucleus is usually spherical in form : it is enclosed 

 in a delicate nuclear membrane (nu.m) and contains, as in 

 Amceba (p. 7) two constituents, the nuclear sap and the 

 chromatin which exhibit far more striking differences than 

 the two constituents of the cell-body. The nuclear sap 

 is a homogeneous semi-fluid substance which forms the 

 ground-work of the nucleus : it resembles the clear cell- 

 protoplasm in its general characters, amongst other things 

 'in being unaffected by dyes. The chromatin (chr) takes the 

 form of a network or sponge-work of very variable form, 

 and is distinguished from all other constituents of the cell 

 by its strong affinity for aniline and other dyes. Frequently 

 one or more minute globular structures, the nucleoli (B, nu'\ 

 occur in the nucleus either connected with the network or 

 lying freely in its meshes : they also have a strong affinity 

 for dyes although they often differ considerably from the 

 chromatin in their micro-chemical reactions. 



