OUTGROWTH OF THE NERVE FIBER 
819 
in the first interval than in the second. Considerable numbers 
of loose cells had by this time moved into the field, so that the 
proximal part of the fiber, which was visible earlier, was then cov- 
ered up, but the five corpuscles were still plainly in view and their 
relative position remained still unaltered. During all this time 
the end of the fiber had been changing its form continually. It 
is inconceivable that the fiber could have been pushed past the 
corpuscles by any force acting from behind. Again, it is impos- 
sible that the five corpuscles could have shifted materially and at 
the same time have retained their same relative position. Nor 
can we account for the movement b}' tension upon the fiber from 
beyond, for such tension would act also upon the corpuscles and 
upon the cells in like manner; and the continual change in the 
form of the end and the lack of any appearance of tension like- 
wise speak against this mode of accounting for the movement. We 
therefore cannot escape the conclusion that the extension of the 
fiber is due to the activity of the enlargement at its end. 
The character of the movement that takes place at the end of 
the fiber is difficult to describe. The filaments in which the fiber 
ends are extremely minute and colorless, showing against their 
colorless surroundings only by difference in refraction. The eye 
perceives, therefore, only with difficult} an actual movement, 
though when an active end is observed for five minutes it will be 
seen to have changed very markedly, so that in making drawings 
one encounters the difficulty of having the object change before 
the outline can be traced. In order to show the change of form 
that does take place, a series of sketches are reproduced in text 
fig. 3, which show the same fiber at intervals of from five to nine 
minutes. The specimen was a portion of the ectoderm with the 
underlying cranial ganglia taken from the branchial region of a 
sylvatica embryo. When first seen the fiber was about 450m 
long. The next day, April 6, 1909, four days after the prepara- 
tion was made, the fiber had increased in length to SOO/x when the 
first sketch was made at 10.50 a.m. The changes shown in the 
sketches took place between 10.50 and 11.37 a.m. The end of 
the fiber was just beyond a red corpuscle, the position of which 
was fixed. This is shown in outline in each figure. It will be 
