THE STRUCTURE OF THE CELL 3 



wall. Their boundaries are usually determined by the pressure of 

 neighbouring cells and other tissue-constituents. They often occur 

 in cysts filled with similar cells. The external cells lie against the 

 wall of the cyst, but the others may be bounded by cells similar 

 to themselves. Mutual pressure gives a characteristic shape to 

 each cell. It has many sub-equal sides. These are nearly flat, 

 except where a cell abuts on the curved wall of the cyst or tubule 

 that contains the cells, or projects freely into the central cavity. 

 In section, most of the spermatogonia therefore appear polygonal 

 (fig. 1). The cell-membrane is exceedingly thin, far below the 

 limit of measurement by the light-microscope; but whether we 

 can actually see a cell-membrane or not in any particular cell, we 

 are compelled by convincing evidence to postulate the existence 

 of a thin surface layer having different characters from the 

 material within. 



Beneath the cell-membrane lies the ground cytoplasm with 

 its inclusions. Spermatogonia commonly contain five kinds of 

 cytoplasmic inclusions that are large enough to be seen with the 

 light-microscope. Three of these appear to be invariably present 

 in ordinary cells. They do not occur in mature red blood- 

 corpuscles of mammals, and all of them do not occur in all 

 spermatozoa ; but they are present in young red blood-corpuscles 

 and also in spermatids, which are young spermatozoa. The three 

 kinds of cytoplasmic inclusions that are present in spermatogonia 

 and in all ordinary cells are the nucleus, mitochondria, and lipid 

 globules. 



It may be thought degrading to the dignity of the nucleus to 

 call it a cytoplasmic inclusion, but logic seems to demand that it 

 should be included under this head. Some authorities would list 

 the 'Golgi apparatus' as a regular constituent of cells, but this 

 name appears to have been given to a variety of different objects, 

 not falling into a single category.^^^- i^- loo, 15, 170, 171. 19. 55, 56, 



133, 22, 113 



Spermatogonia commonly contain two other kinds of cyto- 

 plasmic inclusions, which are of frequent but not universal occur- 

 rence in cells. These are the centriole and idiozome. 



The five kinds of cytoplasmic inclusions will now be briefly 



