THE TYPICAL CELL 



FIG. 2. VARIOUS SPHE- 

 ROIDAL CELLS. 



1, ovuin, from the ovary 

 of a child ; #, spermatocyte ; 

 and 3, spermatid, from the 

 testicle of a rabbit. Hema- 

 tein and eosin. x 750. 



connected with the processes of reproduction by cell division, 

 karyokinesis or mitosis. In fact, as the nucleus is to be considered 

 as the controlling center of cell activity, 

 growth, and development in short, of con- 

 structive metabolism so also is the centro- 

 some, in all probability, to be considered as 

 the dynamic center of the cell, controlling 

 the formation of those mitotic figures which 

 finally result in cell division. 



A small, spheroidal, distinctly chromatic 

 body is also frequently seen in the cyto- 

 plasm in the vicinity of the nucleus. This 

 is the true u nebenkern " of Biitschli, which 

 has been shown by Platner and La Valette 

 St. George to be the remains of the mitotic 

 nuclear spindle. At times it has a dis- 

 tinctly fibrillar structure. 



Protoplasm. The finer structure of pro- 

 toplasm, while certain fundamental facts 



may be readily observed, is open to varied interpretation by differ- 

 ent observers. These interpretations have evolved several theories 

 to explain the minute structure of this substance. It is certain 



that protoplasm, though at times 

 perfectly homogeneous and appar- 

 ently structureless, upon careful 

 examination usually presents a fine 

 reticulum. It is equally certain 

 that this reticulum is not merely 

 the product of coagulation by strong 

 fixing reagents, for it has been fre- 

 quently observed in living cells. 



Protoplasm may therefore be 

 said to consist of a reticular net- 

 work and an intervening, fluid, 

 ground substance. To the former 

 Leydig gave the name spongioplasm, 

 to the latter hyaloplasm.* Neither of these structures is, how- 

 ever, of homogeneous, nor even of constant composition. The 



* Filar mass and interfilar mass, according to Fleming's theory of the fibril- 

 lar structure of protoplasm. The terms mitome and paramitome are also 

 equivalent. 



FIG. 3. EPITHELIAL CELLS FROM THE 



AMPHIBIAN PANCREAS. 



N, nebenkern. Highly magnified. 



(After Matthews.) 



