CELLS AND TISSUES 3 



edoplast may be distinguished; in some cases the plasma membrane 

 appears to be the most superficial layer of the ectoplast (see Figs. 1, 48, 

 and 49). 



In plant cells plastids of one or more types are nearly always present 

 in the cytoplasm. The most conspicuous among these are green chloro- 

 plasts, which are concerned in the fundamental process of carbohydrate 

 synthesis (see Figs. 25 to 29). 



A characteristic constituent of animal cells is the Golgi material, which 

 may appear in the form of small bodies or more or less extensive networks 



Fig. 1. — Living cells as they appear under dark-field illumination, a, cell from hair 

 of squash. The cytoplasm lines the cell and extends in strands through the vacuolar 

 matter; nucleus in lower center. (After Heidenhain, 1897.) b, cell from chick embryo 

 in tissue culture. The cell is adhering to the underside of a cover slip. Near the nucleus 

 are numerous fat globules; note also filamentous chondriosomes. (After Strangeways and 

 Canti, 1927.) 



in the cytoplasm. It is therefore often called the "internal reticular 

 apparatus." It evidently plays a special role in secretion (see Figs. 35 

 and 37). 



A centrosome is found in the majority of animal cells and in those of 

 certain lower plants. It is typically made up of a central granule, or 

 centriole, imbedded in a mass of hyaline or alveolar material known as the 

 centrosphere, but either of these elements may be present alone. During 

 stages of nuclear division the centrosome is surrounded by a conspicuous 

 system of radiating astral rays, collectively called the aster (see Figs. 85 

 and 86). 



Chondriosomes have been demonstrated in the cytoplasm of nearly 

 all plant and animal, groups. These are minute bodies having the form 



