CELL NUCLEUS. 



29 



CELLS AND CELL TISSUES. 



The constituent parts of which an organ is made up are known as 

 tissues. They possess a definite structure, visible with the help of 

 a microscope, and have either the form of cells or of structures 

 derived from cells. Tissues have a function corresponding to their 

 special structure, and this function determines the whole function of 

 the organ. They may, therefore, be regarded as organs of a lower 

 order. The ultimate unit, the organ of the lowest order, or ele- 

 mentary organ,* from which all tissues are derived, is the cell. 

 The essential part of a cell is not, as we have already seen, the 

 membrane or the nucleus, but the protoplasm, with its special 

 molecular arrangement, in which reside the functions of independent 

 movement, of metabolism and of reproduction (fig. 1). 



The nucleus of a cell is either a solid mass of protoplasm or a 

 more fluid structure enclosed by a firm membrane, and may con- 

 tain one or more solid bodies (nucleolus). Different as are the 

 forms which the nucleus may take, it always contains a fluid sub- 

 stance, the nuclear fluid, and a pro- 

 toplasmic substance, the nuclear 

 substance of a special importance 

 for the functions of the nucleus 

 (fig. 17). 



An important and very general 

 property of protoplasm is its 

 power of contractility. The living 

 mass presents, in connection with 

 metabolism, phenomena of move- 

 ment. These movements are not 

 merely confined to the currents of 

 solid particles suspended in the 

 viscous contents of the cell, but 

 are shown also in the change of 



form of the whole cell. If the outer part of the protoplasm has 

 condensed so as to give rise to a cell membrane, i.e., if the cell has 

 acquired a distinct wall, the changes in its form are very much 

 restricted. In other cases the movement shows itself in a quick 

 or slow change in the outer form. The cell in this case manifests 



* Th. Schwann, " Microscopische Untersuchungen iiber die Uebereinstimmung 

 in der Structur und dem Wachsthum der Thiere und Pflanzen." Berlin, 1839. 

 Fr. Leydig, "Lehrbuch der Histologie des menschen und der Thiere." Frank- 

 furt a.M. 1857. 



Fis 17. Different forms of nuclei (after 

 R. Hertwig). a, nucleus from a cell 

 of a Malpighian tubule of a caterpil- 

 lar, b, nucleus of a Heliozoon with 

 a cortical layer and nucleolus in the 

 nuclear fluid. c, nucleus from the 

 egg of a Sea-urchin. Nucleolus im- 

 bedded in a protoplasmic fibrous net- 

 work surrounded by nuclear fluid. 



