6 Bulletin Wisconsin Natural History Society. [Vol. 8, No. 1. 



from the presence of their end organs. The first observations were 

 made on the chick embryo, bnt these were afterward confirmed by 

 experiments on the frog, toad and salamander. 



EXPERIMENTS ON THE CHICK. 



I. The wind bud was removed, usually by electrolysis, after three 

 days of incubation, the egg again closed and allowed to develop from 

 24 hours to five days. At the time of this operation the nerve roots 

 are already established, and extend largely to the inner margin of the 

 wing, but have not penetrated it. There is, therefore, no direct injury 

 to the nervous system. Varying amounts of muscle tissue developed 

 from the three somites from which the wing arises are left by this 

 operation, but in every case it was found that: 



1. The complete destruction of the primordium of any muscle 

 before its innervation results in the complete suppression of the branch 

 of the peripheral nerve leading to it. 



2. The nerve leading to a defective muscle, resulting from the 

 destruction of a portion of its primordium before innervation is always 

 decreased in size, but is usually larger in proportion to the size of the 

 normal muscle than the normal nerve is in proportion to the size of 

 the normal muscle. 



3. The ventral roots and ventral horn are always decreased in 

 size. 



4. Destruction of sensory areas always results in a decrease in 

 the size of the nerve trunk, ganglion, dorsal root and dorsal horn. 



5. All of these defects are due to lack of development and not to 

 degeneration. 



II. One, two or three wing somites were removed from one side 

 when the embryo was about two days old. At this time there is no 

 apparent differentiation of the motor cells in the spinal cord, and no 

 outgrowth of peripheral nerves. Some motor nerve fibers develop in 

 each somite even if all the musculature of that somite is destroyed, 

 but instead of forming a compact nerve root, they at first run freely 

 in the mesenchyme, and later become connected with muscles in an 

 adjacent somite. There is no degeneration. 



Miss Shorey said that from these experiments she concluded that 

 the neuroblasts of the chick are not self-differentiating but that they 

 are wholly dependent for differentiation on the presence of end organs. 

 These end organs are not necessarily those which they normally inner- 



