44 GENERAL CONCEPrS 



two parts and increasing the area of nuclear membrane, will restore 

 optimal conditions. There is some evidence to suggest that the chromo- 

 somes may release a substance or substances which initiates first the 

 nuclear events of prophase and metaphase, and secondly the reactions 

 in the cytoplasm which form a cleavage furrow and bring about the 

 division of the cytoplasm. 



Another theory postulates the initiation of mitosis by a "cell division 

 hormone." The mitoses of the cells of an egg undergoing cleavage occur 

 simultaneously, which suggests that a periodically released hormone may 

 control these divisions. The experiments of Haberlandt indicate that 

 dying cells release a substance which stimulates cell division. He cut a 

 potato in half and examined the cut edge for mitoses. He found that 

 if he cleaned the cut edge to remove all cell debris few mitoses occurred. 

 If he did not clean the cut edge, cell divisions were more frequent, 

 and if he put some mashed cells on the cut edge an even greater number 

 of cell divisions resulted. He concluded that cut potato cells release a 

 "wound hormone" which stimulates cell divisions in adjacent cells. 

 Marshak and Walker were able to prepare an extract of the nuclei of 

 rat liver cells and then to separate this into two fractions. One fraction, 

 when injected into other rats, increased, and the other decreased, the 

 rate of cell divisions in liver cells. 



12. The Study of Cellular Activities 



Despite great differences in size, shape and location in the body, all 

 cells have many metabolic activities in common. Each cell has a host of 

 enzymes which enable it to release energy by converting sugars, fats and 

 proteins to carbon dioxide and water. Each cell synthesizes the structural 

 proteins and enzymes of its own protoplasm. Superimposed on this basic 

 pattern of metabolism common to all cells may be other activities 

 peculiar to each type of cell. For example, muscle cells have special 

 proteins, myosin and actin, which are contractile; particular digestive 

 enzymes are produced by the cells lining the stomach and intestine; and 

 the cells of the pituitary, adrenal and thyroid glands manufacture char- 

 acteristic hormones. 



There are many ways of studying cellular activity and each of these 

 provides useful information about cell morphology and physiology. 

 Living cells suspended in a drop of fluid can be examined under an 

 ordinary microscope or with one equipped with phase contrast lenses 

 (Fig. 3.7). In this way one can study the movement of an ameba or a 

 white blood cell, or the beating of the cilia on a paramecium. Cells from 

 a many-celled animal— a frog, chick or man— can be grown by "tissue 

 culture" for observation over a long period of time. A complex nutritive 

 medium, made of blood plasma, an extract of embryonic tissues and a 

 mixture of vitamins, is prepared and sterilized. A drop of this is placed 

 in a cavity on a special micro slide, the cells to be cultured are added 

 aseptically, and the cavity is sealed with a glass cover slip. After a few 

 days the cells have exhausted one or more of the nutritive materials and 

 must be transferred again to a fresh drop of medium. Cells transferred 



