OUTGROWTH OF THE NERVE FIBER 
791 
will be given here, in order to afford a basis for comparison with 
the protoplasmic filaments formed by the isolated pieces of ner- 
vous tissue. Fortunately the part descriptive of normal develop- 
ment need not occupy very much space, for we now have a large 
mass of facts available in the recent work of Ramon y Cajal 
('07- '08) and in the exhaustive monograph of Held ('09). 
The method which I have used is, in a word, as follows: Small 
pieces of embryonic tissue, taken before the histological differ- 
entiation of nerve fibers has begun, are placed in hanging drops 
of lymph, and the sealed preparations kept under observation for 
a number of days. It is found that the embryonic cells under 
these conditions manifest striking amoeboid activities, which are 
especially pronounced in cells taken from the nervous system, and 
result in such cases in the formation of long threads of hyaliiie 
protoplasm. These fibers bear a perfect morphological re- 
semblance to undoubted nerve fibers found in sections of nor- 
mal embryos of a corresponding stage of development. So strik- 
ing is the similarity between these structures that no hesitancy 
is felt in regarding them as identical with one another. 
This method, which obviously has many possibilities in the 
study of the growth and differentiation of tissues, has two very 
distinct advantages over the methods of investigation usually 
employed. It not only enables one to study the behavior of 
cells and tissues in an unorganized medium free from the influ- 
ences that surround them in the body of the organism, but it 
also renders it possible to keep them under direct continuous 
observation, so that all such developmental processes as involve 
movement and change of form may be seen directly instead of 
having to be inferred from series of preserved specimens taken 
at different stages. While these two advantages have not here- 
tofore been combined in a single mode of procedure, the first 
named has been attained by Loeb ('02) who has embedded pieces 
of tissue, chiefly epidermis, in blocks of agar or clotted blood and 
transplanted them to spaces in the body of living animals. It is 
interesting to note that under these conditions epithehal cells 
undergo changes which apparently resemble closely the activities 
of embryonic cells observed in the present investigation, as a 
comparison of Loeb's figures with my own shows. 
