28 MANUAL OF PHYSIOLOGY. 



II. The Nucleus. Most independent masses of protoplasm, 

 and all highly organized cells, contain one or more nuclei in their 

 substance. The nucleus is in a greater or less degree sharply di- 

 vided off from the surrounding protoplasm. Its presence can 

 generally be made much more conspicuous by treating the cell 

 with various chemical reagents, notably dilute acids and certain 

 dyes. The nucleus is, in the former case, able to resist the action 

 of dilute acetic acid for a much greater length of time than the 

 remainder of the cell, so that it stands out clearly, while the rest 

 becomes quite transparent. In the latter case, magenta (one of 

 the aniline dyes) stains the nucleus sooner and deeper than the 

 protoplasm. Although it has been accredited with special inde- 

 pendent movements, it may safely be said that in comparison with 

 the protoplasm it is not very contractile. Yet it appears to be 

 intimately associated with the vital phenomena of the cell, and 

 may be said to control or initiate the most important activities of 

 the cell, namely, its division. The small size of the nucleus adds 

 greatly to the difficulty of investigating its functions, and much 

 remains to be made out concerning both its structure and proper- 

 ties, although recently considerable progress has been made iu 

 this direction. 



III. The Cell Wall. It has already been stated that the 

 most active form-elements, such as the cells in the earliest stages 

 in the life of an organism (embryonic cells), have no inclosing 

 membrane or cell wall. But in the more advanced stages of cell 



O 



life we find this second form of protoplasmic differentiation to be 

 common enough. In animal cells the limiting membrane has 

 never the same importance as the cell wall in vegetable tissues, 

 where some of the principal textures may be traced to a direct 

 modification of the cell wall, still recognizable as such. When- 

 ever such a limiting membrane does exist, it is always formed by 

 the outer layers of protoplasm undergoing changes so as to become 

 of greater consistence. In the animal tissues the protoplasmic 

 units form various structures, which, however, do not hold the 

 relation to them of limiting membranes, but rather give the idea 

 of lying between the cells. Hence, in one large group of tissues, 



