78 PROTOPLASM 



It is not always an easy task to distinguish the types of cells 

 which appear in culture. The difficulty is due to the fact that 

 embryonic cells are less differentiated than are mature ones. 

 Growth in culture tends toward dedifferentiation, while growth 

 in the body tends toward differentiation. The extent of the 

 dedifferentiation in culture is in part dependent upon the nature 

 of the culture medium. Certain types of cells become trans- 

 formed into other types. This tendency toward transformation 

 is known as polymorphism and occurs also normally within the 

 body but is accentuated in culture. 



Polymorphism, or the capacity to exist in many forms, is very 

 characteristic of tissue-culture cells. It is a property that one 





1 



Fig. 63. — Fibroblasts showing (left) aggregation of fat globules; and (right) 



worm-shaped mitochondria. 



might well expect cells to have when they are embryonic, for 

 their destiny is then a vague and distant thing. Whether they 

 are to become nerve cells, blood leucocytes, fibroblasts, or 

 macrophages is not yet fixed, or, if partly predetermined, develop- 

 ment in that direction has not progressed so far but that their 

 fate may still be altered. Those two factors which primarily 

 determine the destiny of cells are origin and environment; the 

 latter appears to predominate. The problem is one of wide 

 occurrence in nature. In the eastern tropics, the pitcher plant 

 Nepenthes forms low rosettes with many pitchers when growing 

 on wet, boggy ground. On dry, solid ground, the plant grows 

 as a long vine and forms no pitchers. The difference may be due 

 simply to moisture, though salts are probably a determining 

 factor. A single tropical species of the genus Vaccinium may, 

 in the same locality, occur as a tree, shrub, or vine and be 

 terrestrial or epiphytic (i.e., on the soil or off the soil on other 

 plants). The causes of this polymorphism are unknown. 



