168 J. M. D. OLMSTED 
as a proliferation from the old nerve cord itself. All the sections 
I have examined tend to support this view, since the old nerve 
cord presents the same appearance in all stages of regeneration, 
and the new nervous material is absolutely distinct from the old. 
If this view is correct, the fact that one eye appears earlier than 
the other, and that this eye is on the same side of the body (in 
these triangular pieces) as the old nerve cord, may be explained 
by supposing that the differentiation of nervous tissue is a pro- 
gressive process, which begins at the anterior end of the fragment 
of old nerve cord and completes the nerve cord anteriorly on 
that side first; then, after the formation of the portion of the 
brain on the same side, the portion of the opposite side is produced, 
and after that, the nerve cord of the opposite side, which finally 
joins with the posterior end of the old nerve cord. The side 
upon which the brain is first differentiated would, of course, be 
the first to develop an eye. 
Morgan (98) was unable to find a pharynx in any of his 
regenerated triangular pieces. In one case in my experiments 
(fig. 7) a pharynx was fully developed. It was formed in the old 
tissue as in the ordinary course of regeneration, but its position 
was anomalous. The opening through which it projected lay 
on the dorsal surface between the two eyes, and the pharynx 
itself was directed forwards in the long axis of the regenerated 
worm. Often when disturbed the animal thrust out the pharynx 
beyond the head and then drew it back. An attempt to feed 
this animal with liver to prolong its life was unsuccessful. No 
food could be seen to enter the pharynx, though it executed, 
rather feebly, the feeding movements. The worm began to 
disintegrate on the 21st day, and an attempt to fix and section 
it was made. In a second specimen no pharynx was _ visible 
externally, but on the 15th day, while the worm was still vigorous, 
it was killed and fixed, and later sectioned. The sections re- 
vealed that such an organ had begun to develop between the 
eyes (fig. 9). The position of the pharynx is not quite so extra- 
ordinary as it at first seems. Had the piece regenerated a 
head along the anterior and a tail along the posterior cut, the 
position of the pharynx would have been nearly that which is 
