EMBRYOLOGY 1 83 



which normally would become brain cells, if they are transplanted to the 

 belly region of the embryo, are "demoted," so to speak, and become merely 

 belly epidermis. Furthermore, it can be shown that, if placed in the proper 

 location, belly epidermis instead of becoming brain, may become muscle 

 tissue, or even a part of the kidney. 



Such experiments command our attention. They show that, although 

 cells in early developmental stages may normally have a very definite 

 presumptive fate, this fate can be changed when the position of the cells 

 in question is changed. Hence, we can be certain, that in the young amphib- 

 ian embryo determination of parts has not, at least in all cases, been ir- 

 revocably established. The fate of a certain group of cells can be altered 

 by changing their location in the embryo. Accordingly, it appears that in 

 some manner the immediate surroundings of cells are of basic importance 

 in determining their significance in development. 



It should be stated, however, that the alteration in the fate of embryonic 

 cells can be brought about only in young stages of development. Quite early 

 in embryonic development a time is reached at which the fate of the 

 embryonic cells becomes irrevocably determined. When this times comes, 

 epidermis from the belly region, if transplanted to the brain region then 

 "refuses" to be influenced by the new surroundings and "insists" on de- 

 veloping into skin epidermis. Hence, when such cells are transplanted, we 

 will have formed a little abnormal knot of epidermis cluttering up the 

 brain region. Or, conversely, if presumptive brain tissue be transplanted to 

 the body wall in the belly region, a bit of abnormal nervous tissue will later 

 be found trying to develop on the belly wall. Such results have been ob- 

 tained many times in amphibian embryos, and the experiments are rela- 

 tively easy to perform. 



The question arises as to what forces are responsible for setting up this 

 irrevocable determination of regions in the young embryo at a certain 

 stage in development? This question is too large and too involved to take 

 up in detail in a discussion such as the present one. But, permit me to 

 give a few hints with regard to what is going on. 



In one special region of a developing amphibian egg — a region which 

 biologists will recognize as the dorsal lip of the blastopore — is situated what 

 Professor Spemann has called the "center of organization." An extensive 

 series of experiments have shown that this center of organization is re- 

 sponsible, to a great extent, for the organization of cells forming some of 

 the main axial organs of the body, such as the brain, the spinal cord, and 

 the predecessor of the spinal column, which is called the notochord. Once 

 this center of organization has become active, then regions under its in- 

 fluence become irrevocably determined. 



I shall give you just one example, of the manner in which the center of 

 organization appears to operate. If cells are taken from this center and trans- 

 planted so that they can influence presumptive skin epidermis in the ventral 



