48 - The Cell 



Likewise the problem of how the division 

 furrow develops energy for cleaving the ani- 

 mal cell into two daughters is receiving con- 

 siderable attention. Here, however, two more 

 or less opposite hypotheses are being tested: 

 (I) that furrowing results from an expansion 

 of the cell surface (viewpoint of M. M. Swann 

 and M. Mitchison, in Great Britain); and (2) 

 that furrowing results from the contraction of 

 a peripheral girdle of strongly gelled (p. 94) 

 cytoplasm that forms around the equator of 

 the cell during telophase (viewpoint of W. 

 H. Lewis and D. A. Marsland, in the United 

 States). At present, final decision on this 

 problem cannot be made, although recent 

 developments tend to favor the contraction 

 viewpoint. 



Control of Cell Division. One very impor- 

 tant and difficult problem in biology is to 

 ascertain precisely how the multiplication of 

 cells is controlled within the body so that 

 the number of cells in each part is appropri- 

 ate to the requirements of the body as a 

 whole. In the human body, for example, mi- 

 totic activity remains at a high level in the 

 different parts of the embryo, but as develop- 

 ment proceeds, mitosis becomes less and less 

 frequent. At maturity, indeed, most of the 

 cells have stopped dividing. Mitotic activity 

 becomes confined to certain local i/ed regions, 

 such as the skin, where the epithelial cells 

 continue to multiply, replacing the surface 

 layers as they wear away; in the gonads (p. 

 380), where the many prospective gametes are 

 being formed; in wounds, while the processes 

 of repair are going on; and in blood-forming 

 and certain other tissues. 



The Cancer Problem. Little is known about 

 the factors that regulate cell division, but 

 usually they are most effective in controlling 

 the population of each kind of cell in the dif- 

 ferent organs of various animals and plants. 

 On rare occasion, however, something goes 

 wrong and the cells in some part of the body 

 begin dividing without restraint. This results 

 in an abnormally large and improperly or- 

 ganized mass of cells called a tumor. Benign 

 types of tumor, which do not display a very 



high level of mitotic activity and which tend 

 to become encapsulated by the surrounding 

 tissues of the host, are not very serious. 

 Eventually the benign tumor tends to stop 

 growing and usually it is not difficult to re- 

 move surgically. But malignant tumors, or 

 cancers, possess a very high mitotic rate and 

 an unlimited capacity to spread at the expense 

 of any surrounding tissues. Moreover, small 

 groups of the cancerous cells may become de- 

 tached from the main mass and be carried by 

 the circulatory system to some other part of 

 the body, where a secondary cancer, or metas- 

 tasis, will start. Consequently, it is most im- 

 portant that a malignant tumor be detected 

 and removed as soon as possible, before it has 

 invaded some vital region, and before it has 

 metastasized. Among higher animals gener- 

 allv, two main types of malignancies are dis- 

 tinguished: carcinomas, in which the can- 

 cerous cells are of an epithelial nature; and 

 sarcomas, in which the cells are of a connec- 

 tive tissue type. 



The causation of cancerous growths is a 

 complex problem, especially since not all 

 types originate in the same way. One type, 

 the Rous sarcoma in chickens, appears to be 

 initiated by a fdtrable virus. This type, there- 

 lore, can be transmitted to a healthy bird 

 merely by injecting cell-free fluid from a dis- 

 eased individual. The transmission of other 

 tumors, on the other hand, involves trans- 

 planting some of the cancerous cells. Also, it 

 is known that certain bacteria are capable of 

 inducing tumorous growths in plants, al- 

 though once induced, such tumors can con- 

 tinue to grow even in the absence of the bac- 

 teria. Certain malignant tumors in animals 

 can be induced by chemical compounds 

 (originally isolated from tar), and once in- 

 duced, such cancers likewise continue to grow 

 after the carcinogenic compound has been 

 removed. All in all, therefore, it seems prob- 

 able that cancerous cells develop some in- 

 trinsic difference, possibly a change in their 

 genie material, that makes them insensitive 

 to hormonal (p. 400) and other factors that 

 normally limit the multiplication of body 



