STRUCTURE AND PHYSIOLOGY OF ANIMALS 



17 



the second, more fluid substance in its meshes (Fig. 4). Apparently 

 both of these types of structure occur in the protoplasm in different 

 cases, sometimes even in intimate association. 



To a particle of protoplasm, typically containing a nucleus in its 

 interior, constituting the entire body of such a simple organism as 

 Amoeba, and forming one of the constituent elements of which a 

 higher plant or animal is made up, the term cell is applied. The 

 word was first employed in reference to the microscopic structure 

 of plants, in connection with which it is much more appropriate 

 than in connection with the microscopic structure of animals ; for 

 a plant-cell has, nearly always, a definite, firm, enclosing envelope 

 or cell-wall (Fig. 5, 7, c.w) a structure which is only exceptionally 

 present in the case of animals. In the interior of the cell-protoplasm, 

 or cytoplasm, is a body termed the nucleus, similar to the nucleus 

 of Amoeba, and usually of rounded shape, with the appearance of 

 being enclosed in a thin 

 nuclear membrane (A, 

 nu.m), perforated by 

 numerous minute aper- 

 tures. The nucleus con- 

 tains a very complex 

 protoplasmic material 

 termed chromatin, which 

 differs from the cyto- 

 plasm in its strong affinity 

 for certain dyes or 

 staining agents and also 

 in containing phosphorus. 

 Chromatin is shown, by 

 its universal occurrence 

 and its persistence throughout the changes which the cell under- 

 goes, to be the most essential constituent of the nucleus. 



In addition to the chromatin the nucleus contains linin, nuclear 

 sap, and plastin. The linin is of the same character as the reticular 

 or alveolar cytoplasm, and the nuclear sap corresponds to the more 

 watery portions of the latter. The plastin is related to chromatin 

 with regard to affinity to staining agents, but differs from it definitely 

 in that and other respects. The arrangement of these constituents 

 of the nucleus differs greatly in different nuclei and in the same 

 nucleus in different phases. The chromatin, in the resting con- 

 dition of the nucleus, i.e. when cell-division is not in progress, may 

 be distributed as granules throughout, with, or more rarely without, 

 a definite relationship with the reticulum or alveoli. In a typical 

 resting nucleus the chromatin granules, or chromioles, are strung 

 along the threads of the linin reticulum or in the interspaces between 

 the alveoli so that the chromatin in a stained nucleus stands out as 

 a conspicuous network. But in the nuclei of many unicellular 



VOL. i c 



FIG. 4. Diagram to illustrate the reticular theory of 

 protoplasm. (After Dahlgren and Kepner.) 



