158 EMBRYOLOGY 



learning period give us a very clear answer. Young frog embryos exposed to 

 a solution of chloretone before any muscular movements set in will remain 

 quiescent for several days, during which time they develop perfectly normally. 

 If we now return them to their normal environment the anesthetic washes 

 out and within a few minutes they are swimming vigorously. It is clear, 

 therefore, that all the connections and all the pathways necessary for the 

 reflex movements for swimming have been laid down in the complete absence 

 of any function. 



As a matter of fact, structures such as the limb will develop perfectly 

 without any nerves, and therefore with no function. The best example, 

 perhaps, is the case of a transplant of an embryonic limb bud of the chick 

 embryo into the body cavity. Here it is well nourished and the muscles and 

 bones of the limb develop perfectly in spite of the fact that no spinal nerves 

 ever enter the limb. In embryonic development, therefore, the morphological 

 changes in both the nervous system and the limb take place without reference 

 to function. The same situation is not true in the adult, however. It is a 

 commonly known fact that in the adult muscles without nerves will degen- 

 erate. It is not quite so common a fact that the development of a limb by 

 regeneration in a young salamander larva must be accompanied by the 

 regeneration of the nerve. In a larva which normally regenerates a limb 

 after amputation, this regeneration can be completely inhibited if the limb 

 nerves are cut. Under these conditions, instead of regeneration, complete 

 degeneration of the limb takes place. 



These experiments on the development of the nervous system show that 

 the principles of early development apply to the later stages as well. Both the 

 spinal cord and the spinal nerves show a labile organization. Early embryonic 

 induction is illustrated by the effect of the limb on the differentiation of the 

 motor region of the spinal cord and on the differentiation of cells in the 

 spinal ganglia. In addition, the modification of any one of the third, fourth, 

 and fifth nerves to take the place of all three must be considered as a case of 

 embryonic induction. The muscles of the limb induce changes in the nerves. 



We have found, in addition, a new principle of directed movements of 

 cell outgrowths. This phenomenon was not considered earlier, although 

 during gastrulation there is an extensive migration of cells, just as in the 

 development of nerves there is a migration of nerve fibers. The mechanism 

 of directed cell movements is not clear, and the theories are not completely 

 convincing. 



