OUTGROWTH OF THE NERVE FIBER 
813 
the phenomena which have been observed may not have their 
counterpart in the embryonic body, but fortunately the careful 
comparison with what takes place in the latter enables us to dis- 
criminate with a fair degree of accuracy between the abnormal 
and the normal. 
Description of the behavior of nervous tissues 
The early changes which isolated pieces of medullary cord un- 
dergo are not very different from those seen, for instance, in pieces 
of mesoderm. There is merely greater protoplasmic activity. 
This frequently results in the separation of numerous cells which 
may move off individually from the main mass of tissue (figs. 
10, 11, 16 and 27), or it may result in the formation of sheets of 
cells, one layer thick, which form a more or less complete fringe 
around the main mass (fig. 16). The formation of rings, as de- 
scribed above, is also frequent. These changes begin on the day on 
which the tissue is implanted, and m.ay continue for several weeks, 
although the first week — usually the first four days — witnesses 
practically all the essential changes that take place. Observed 
from time to time, these cells may be seen to change their shape 
and their position relative to one another. Frequently they show 
anastomoses, though in a great many cases apparent continuity 
is often found on continued observation to be merely due to very 
close contact which may later be relinquished. 
The striking peculiarities of this tissue have never been observed 
earlier than on the next day after isolating. Then it is that the 
long filaments, identical in form with the nerve fiber of the normal 
embryo, begin to appear. Two and three days after implantation 
they show their greatest activity. After the fifth day they are 
usually no longer to be found. 
It will be well to begin with a description of one of the most 
striking tj^pical cases and to consider the more aberrant forms 
afterward. In the case chosen for this purpose (experiment 137) 
the tissue was isolated from an embryo of R. palustris and the 
l3^mph was taken from an adult R. pipiens. The day after the 
preparation was made, there appeared on one side of the main 
