812 
KOSS GRANVILLE HARRISON 
characteristic structure of the muscle fiber, the striated fibril, is 
formed with the alternating dark and light bands as well as 
Krause's membrane plainly visible. In many specimens in which 
the myotomes have been taken out with the medullary cord, 
muscle twitchings have been observed, beginning to occur the 
next day after isolation, and continuing sometimes up to the sixth 
day. No contractions have ever been witnessed in muscle com- 
pletely isolated from nervous tissue. 
Characteristic pigment cells have been observed a considerable 
number of times to arise apparently from pieces of the medullary 
tube from which large numbers of single cells had separated. 
But only fragmentary information is at present available re- 
garding the development of t?iese cells. In one case, which 
was under observation for several days, it was found that the pig- 
ment first arose as a round mass of granules lying just to one side 
of the nucleus. This gradually increased in size and then the 
pigment granules became scattered through the cj^toplasm. In 
the meantime the yolk was almost entirely absorbed. After the 
cells are fully differentiated, observations from day to day showed 
that the individual cells changed slightly in form (figs. 24-26, a). 
While the evidence is by no means conclusive, especially since no 
great care was taken to exclude the presence of mesoderm cells, 
the fact that pigment cells were frequently formed from pieces 
of medullary cord, suggests the possibility that these cells may 
normally take origin in part from this source, most likely from the 
ganglion crest. This suggestion is borne out by the fact that pieces 
of medullary cord or cranial ganglia when transplanted to various 
regions of the embryonic body often break down and give rise to 
large numbers of pigment cells. 
The above observations are recorded here primarily for the 
purpose of showing that the mode of procedure employed in the 
experiments permits the characteristic difi^erentiation of various 
tissues to take place. It is of importance to establish this fact 
in order that the interpretation which has been given the obser- 
vations upon the behavior of nervous tissue under these conditions 
may not be called into question as being based upon something 
entirely abnormal. It must of course be admitted that some of 
