Noy.) ' DOUBLE-HEADED FORMS. 341 
ness of ganglionic cells (Fig. 5), while in specimens four days 
old no difference from the normal can be detected (Fig. 6). 
More detailed changes are as follows: At the end of 
twenty-four hours the new cells have increased greatly and have 
quite filled up the cut surface, covering over the alimentary 
tube, etc. The cells have increased in size ; they are oval or 
oblong in shape, and exhibit vesicular nuclei. They inclose 
the cut ends of the nervous cords and lie often in parallel 
rows, the long axis of the cells being placed in the horizontal 
plane. The most considerable group of cells is at the cut 
extremity and fibers are already visible between these cells. 
It cannot be positively stated that these cells have not migrated 
inwards among the old fibers and that these latter are the 
objects seen between them. But on the other hand, certain 
cells which lie to either side and just above the cords show 
similar fibers. It would be difficult to account for the pres- 
ence of these upon the supposition that they were pre-existent ; 
it appears more probable that they are newly formed. This 
conclusion is impressed upon one the more as these fibers are 
less distinctly linear and wavy than those in the cords them- 
selves. Many of them are indeed short and granular, and 
some may not be fibers at all, but perhaps a sort of granular 
intercellular substance. This material is, however, not present 
in other places between cells; and it seems limited to the 
region of the old nerve cords. It is conceivable that the 
granules are derived from young and immature fibers altered 
by the fixing and hardening process. 
These cells present the characters of neuroblasts, and are 
doubtless destined to compose the reproduced cephalic nervous 
structures (Fig. 7). 
The histology of the growth of the nervous ganglia in the 
double-headed is not distinct from that of the simple forms. 
The gross anatomical result is, however, quite different. 
Although ganglia, two in number, connected by a commissure 
and closely resembling the normal structures, come to be devel- 
oped, each is in association, not with a pair of nerve cords, but 
with a single cord (Fig. 8). The eye-spots bear about the same 
relation to the ganglia as they do in animals with single heads. 
