STRUCTURE OF THE CELL. 



As mitochondria are described constituents of the cytoplasm of varying 

 form and size, but of such constancy, as to be regarded as recognized parts 

 of every cell. Mostly very minute, they appear as granules or as rods, 

 either short or long, straight or curved, and probably include objects of 

 different nature, whose true function and significance are not now understood. 



While our knowledge as to the nature and purpose of mitochondria is 

 still very limited, certain it is that these bodies are present in every' type 

 of cell and are destined to become of increasing importance in the study of 

 the structure and activities of the cell. 



Although the animal cell very seldom is enclosed by a definite cell-wall, 

 the surface of cell is invested by a delicate pellicle of fatty or lipoid material. 

 As the result of the intricate physico-chemical activities within the living 

 substance of the cell, alterations in the surface tension of the pellicle are 

 induced, which phenomena are known as amoeboid movements. 



The terms exoplasm and endoplasm are employed to designate the 

 homogeneous peripheral and granular central zones of the cell-body respec- 

 tively. Since a more or less pronounced reticular arrangement of the active 

 constituent of the cytoplasm is widely encountered in mature cells, this con- 

 dition may serve as the basis of the description of the morphology of the 

 typical cell. Examination of suitably prepared preparations with adequate 

 lenses shows the cytoplasm of many cells to contain a meshwork composed 

 of delicate threads and plates of the more active substance, the spongioplasm 

 (also called mitome or they/ar mass). Although conspicuous only after 

 appropriate staining, threads of spongioplasm may at times be seen in the 

 unstained and living cell, thereby proving that such structural details are 

 not artefacts due to the action of reagents upon the albuminous substances 

 of the cytoplasm. 



The interstices of the meshwork are filled with a clear, more or less 

 homogeneous semifluid substance to which the name of hyaloplasm (also 

 paraplasm, paramitome or inter- 

 filar mass), has been applied. 7> 

 Embedded within the hyalo- A 

 plasm, a variable amount of 

 foreign substances is frequently 

 present. These include parti- 

 cles of oil, pigment, secretory 

 products and other extraneous 

 materials, which, while of pos- 

 sible importance in fulfilling the 

 purpose of the cell, are not 

 among its essential morpholog- 

 ical constituents. These sub- 

 stances, which are inert and 

 play a subsidiary role in the vital 

 activity of the cell, are termed 

 collectively metaplasm. 



The Nucleus. This, the 

 second essential constituent of 



the cell, usually appears as a sharply defined spherical or ellipsoidal body, 

 which, in stained preparations, is conspicuous on account of its deeper color. 

 Since the nucleus is the nutritive, as well as the reproductive, organ of the 

 cell, the fact that this part of the cell is relatively large in young and actively 

 growing elements is readily explained. While in a general way the nucleus 



FIG. 3. Spermatogenic cells, showing variations in 

 the condition and the arrangement of the constituents of 

 the cytoplasm and the nucleus ; the centrosomes are seen 

 within the cytoplasm close to the nucleus. A, from the 

 guinea-pig, X 1675 (Meves)\ B, from the cat, X 680 (von 

 Lenhossek). 



