62 



MINUTE STRUCTURE OF CELLS 



LESS. 



direct our attention. First of all, however, it will be neces- 

 sary to describe the exact microscopic structure of cells and 

 their nuclei as far as it is known at present. 





chr 



nu.m 



-c.b 



FIG. 9. A, Cell from the genital ridge of a young salamander, 

 showing cell-membrane (c. m), protoplasm or cell-body (c. b) with 

 directive sphere (s) and central particle (<:), and nucleus with membrane 

 (nu. m) and irregular network of chromatin (chr). 



B. Cell from the immature stamen of a lily, showing cell-wall (c. 7t), 

 protoplasm with two directive spheres (s), and nucleus as in A. 



Both figures very highly magnified. 



(A, from a drawing by Mr. J. E. S. Moore : B, after Guignard.) 



There seems to be a good deal 01 variation in the precise 

 structure of various animal and plant cells, but the more 

 recent researches show that in the cell-body or protoplasm 

 (Fig. 9, c. b) two constituents may be distinguished, a clear 

 semi-fluid substance, traversed by a delicate sponge-work. 

 Now under the microscope the whole cell is not seen at 

 once but only an optical section of it, that is all the 

 parts which are in focus at one time : by altering the 

 focus we view the object at successive depths, each view 

 being practically a slice parallel to the lenses of the 

 instrument. This being the case, protoplasm presents the 

 microscopic appearance of a clear or slightly granular 



