36 



THE FOUNDATION 



Part I 



protoplasm called the centrosphere and at its center are either one or two 

 minute, deeply staining bodies, the centrioles. During the interphase of the 

 cell the centrosome is almost always located just outside the nuclear mem- 

 brane (Fig. 3.4). It plays an important part in cell division and at that time 

 divides into two parts from each of which rays extend stimulating a star. 

 Centrosomes have been found in practically all animal cells except nerve 

 cells, but are not present in those of higher plants. Chondriosomes (mito- 

 chondria) are threadlike or granular bodies (lipoproteins) scattered through 

 the cytoplasm, visible in specially treated cells and sometimes in living ones 

 (Fig. 3.4). It is generally agreed that they are physiologically important 

 although details of their function are unknown; in actively secreting cells 

 they increase in size and number. The Golgi substance is an irregular net- 

 work located near the nucleus, first discovered by Golgi, an Italian physician 

 (1898), in nerve cells and later found in almost all the cells of vertebrates 

 and in many invertebrates, especially in glands. Its nature continues to 

 be debated. Fibrillae are fine threads that extend in a definite direction in 

 the cell and may have a supporting, conducting, or contractile function (Fig. 

 3.6). CiHa and flagella are thin cytoplasmic processes extending from the 



Fig. 3.6. Extremely minute fibrils stretched by a 

 microdissecting needle (black spot) pulling out one side 

 of the living cell (a malarial parasite, Plasmodium). 

 (From Seifriz. Courtesy, de Robertis: General Cytol- 

 ogy. Philadelphia, W. B. Saunders Co., 1949.) 



surface of the cell and are used in locomotion or to create currents of fluid. 

 Flagella are relatively long; there are few of them to a cell and different 

 ones lash independently. One group of protozoans, the flagellates, are so called 

 because they swim by means of flagella. Cilia are short and there are many 

 on one cell. They move in unison, rhythmically. Paramecium is the most 

 familiar ciliated protozoan though there are many others. In multicellular 

 animals surfaces are often covered with ciliated cells: the lining of the human 

 trachea, the gills of clams, the gullet of a frog. Gills of fresh, as well as salt 

 water clams, are good material for the study of ciliary movement. 



Nonliving Cell-inclusions. In animal cells the most abundant of these 

 is stored food: yolk granules and oil globules in eggs, glycogen in other cells 

 (Fig. 3.4). In gland cells the materials to be secreted are often held in the 



