6 EMBRYOLOGY 



stimulate cell division in other resting cells? Cells in the adult divide slowly, 

 if at all. Until it is fertilized the egg is in a condition similar to that of adult 

 cells. Extraction of sperm substances might be a way of getting a chemical 

 stimulant for cell division in adult cells. If we could isolate these substances 

 which stimulate cell division, could they be applied to wounds to increase 

 the rate of healing? Perhaps they could be used to control the size of the 

 structures of the body. 



Then, on the other hand, what prevents cell division in the unfertilized 

 egg? This is another important problem — how to stop cell division, or to 

 control it by a chemical inhibitor when it gets out of hand. If we could find 

 the conditions for stopping cell division, these conditions could be applied 

 to the growth of abnormal cells — tumors and cancers. In the early days of 

 the use of X rays, researchers found that irradiation would prevent the 

 cleavage of fertilized eggs. Further research has revealed that regeneration 



.BRAIN 



EGG AT EARLY GASTRULA STAGE 



Fig. 



EGG CONSTRICTED 



2. Left: An amphibian egg in a very early stage of development is 



tied through the center with a fine thread. This pinches the developing egg 

 almost in two. Right: About a week later the ligature has been removed and 

 the embryo possesses two complete heads joined to a common trunk. How can 

 two heads develop from a group of cells which would normally produce one? 



