92 GENERAL BIOLOGY 



rods or network close to the nucleus, but associated especially with the 

 dense protoplasm which surrounds the centrosome. In epithelial cells 

 the network lies close to the free ends of the cells. The Golgi apparatus 

 is probably found in all animal cells, though little is as yet known about 

 it, except that there is a continuity from parent-cell to daughter-cells 

 by a sort of mitotic division of it quite similar to the regular chromosome 

 division. Prolonged treatment with osmic acid will make the Golgi 

 apparatus visible. 



Plastids are differentiated portions of protoplasm representing cer- 

 tain regions in which physiological processes are localized. They are 

 quite common in plants and protozoa. In the former they are usually 

 colored, such as the chloroplasts which are the chlorophyl-carrying or- 

 gans. Each kind of plastid is supposed to serve a separate type of func- 

 tion. 



Attraction sphere and Centrosome. These may be quite conspicu- 



Fig. 28. Golgi Apparatus in Epidermal Cells. 



a. Golgi network beside the nucleus in cell of a horse. 



b. Same in skin of cat, but broken into small rods around the mitotic figure 

 in the large central cell. (After Deinecka.) 



ous although it is not known whether they are important or not in cell- 

 division, shortly to be described. 



Paraplasmic substances. These are the foreign substances which 

 can be seen in the cytoplasm, but which have not become part of the 

 living cell itself. Such are granules of pigment or calcium, fat globules, 

 various vacuoles filled with fluid, etc. 



"It is clear that the construction of the cell is highly specialized in 

 most cases for the function which it is to carry out, and that it is sup- 

 plied with the most perfect mechanisms for these purposes. Some of 

 these are evident in the form of contracted bands in the protoplasm, or 

 in long, nerve processes ( ) like electric wires care- 



fully insulated by sheaths of fatty material, or in mobile cilia which me- 

 chanically perform duties in the transportation of foreign particles. In 

 others, the tools of their trade are recognizable in the form of the 

 granules which seem to prepare ferments by which the chemical pro- 

 cesses which the cells effect are carried out. While these are visible in 



