790 
ROSS GRANVILLE HARRISON 
nized, however, that in all of the first experiments the nerve fibers 
had developed in surroundings composed of living organized 
tissues, and that the possibility of the latter contributing organ- 
ized material to the nerve elements, stood in the way of rigorous 
proof of the view that the nerve fiber was entirely the product of 
the nerve center. The really crucial experiment remained to be 
performed, and that was to test the power of the nerve centers 
to form nerve fibers within some foreign medium, which could 
not by any possibility be suspected of contributing organized 
protoplasm to them. 
Two lines of experimentation were taken up with this end in 
view. The one was to introduce small pieces of clotted blood 
into the embryo, in the path of the developing nerves. This 
gave positive results, in that nerve fibers were found several days 
after the operation, extending from the medullary cord into the 
blood clot, and the sole possible disturbing factor in these ex- 
periments was the presence of scattered embryonic cells, which 
began to organize the clot within two days after its transplanta- 
tion (Harrison '10). 
The second line of experimentation, which consisted in the 
isolation of pieces of living tissue in unorganized media, gave con- 
siderable difficulty at first, but in the spring of 1907 a method was 
finally devised, which satisfactorily accomplished the purpose. 
The present paper contains a complete account of these ex- 
periments, which have been described previously in a preliminary 
notice. 4 In addition, a brief description of the early develop- 
ment of the nerve elements in the normal amphibian embryo 
* The first of these experiments were made in the Anatomical Laboratory of the 
Johns Hopkins University. After my removal to Yale University they were con- 
tinued during the seasons of 1908 and 1909 in the Sheffield Biological Laboratory. 
The repetition of the work gave results which not only confirmed those of the first 
season, but which also met many possible objections that might have been raised 
against the original experiments. The preparations obtained during the second 
season's work were, on the whole, much more convincing than those of the first, 
and they have been used almost exclusiyely in making the illustrations for the 
present paper. The first account of the work was given in a paper before the 
Society for Experimental Biology and Medicine in May 1907, and later the results 
were incorporated in a lecture before the Harvey Society of New York, in March 
1908. 
