CHARACTERISTICS: RELATION TO ENVIRONMENT 235 



But this condition in which any cell can produce any one 

 of many different parts of the body, depending on circum- 

 stances, may continue Into a rather late period of develop- 

 ment, particularly in the vertebrates. At a certain time the 

 egg of such a creature as the frog has become a mass of 

 small cells. Under normal conditions, when we examine this 

 mass, we can predict what part of the adult each part will 

 produce. Cells here will produce the brain, there at the 

 sides the eyes, here the ear, there the spinal cord, here 

 parts of the skin. If we leave the egg to itself, these are 

 Indeed the parts that will be produced. 



But this is not because each cell can produce only that 

 part and nothing else. For it is possible^to cut off a disk of 

 cells from the egg and turn it around, so that what was In 

 front Is now behind, what was right is now left. And now 

 we find that all the cells change their method of action. 

 The cells that would have produced brain now produce 

 skin; what would have yielded skin now yields brain or 

 eyes; what would have given spinal cord now produces cere- 

 bral hemispheres. What happens Is that from a certain spot 

 on the egg — a recognizable spot — an organizing influence 

 starts out, so that this spot Is known as the organizer, or 

 the organization center. This organizing influence, what- 

 ever its nature, creeps from cell to cell, causing each cell 

 to alter internally — through the interaction of its genes 

 and cytoplasm — In such a way as to produce the structures 

 of the embryo. Each cell that Is reached later transforms 

 in such a way as to fit the cells that have gone before — In 

 such way as to make the next proper part in the pattern of 

 the body. At a certain point the cells transform Into spinal 

 cord, the next ones into medulla, those next into midbrain, 

 the next into forebrain, those at the sides Into eyes, farther 

 forward Into skin. This organizing influence passes through, 

 in the same way, whichever cells are present, so that it is 

 clear that any of the cells can produce any of the parts that 



