Form Regulation in Harenactis attenuata 389 
’ 
reasons, to the “tentacle-forming stimulus” than cells slightly 
more distant from the cut. If such is the case the localization of 
a tentacle-forming region on the line of union may be followed 
by more rapid growth on the two sides of the line of union than at 
the line of union itself. Under such conditions the line of union 
might produce the same result as a fold dividing the tentacle- 
primordium and a distinct small tentacle might appear on each 
side of it. Later fusion of the two tentacles might occur in con- 
sequence of correlative change in the constitution of the cells 
of the line of union. 
But whatever the exact process in a particular case, all of these 
forked and branched tentacles are the result of fusion of localized 
regions of growth in consequence of the obliteration of regions of 
less rapid or inhibited growth between them. Such tentacles are 
then, properly speaking, not forked or branched but rather par- 
tially fused tentacles. 
One tentacle with oral and aboral portions, found on the ring 
shown in Figs. 8 and g, is of particular interest. Both of the 
tentacles indicated by the letter a in Fig. 8 are apparently oral 
tentacles, but each of them has what we may call an aboral branch 
at about the lower third of its length. The plane of section 
in Fig. 6 passes through one of these two tentacles, this 
being the one which chiefly concerns us at present—the tentacle 
appears in the left half of Fig. 9. This tentacle began its develop- 
ment as two distinct tentacles, one on each side of the line of union. 
The oral part which is the longer of the two distal branches ap- 
peared somewhat earlier than the aboral. Later, as the diameter 
of the two growing tentacles increased, fusion occurred, and since 
the oral part was the larger, the aboral part became what seems 
to be a shorter branch of it and, in consequence of continued 
growth of the basal fused region, came to occupy the position 
shown in Fig. 9. ‘The line of union between oral and aboral end 
of the piece is represented in this tentacle by the region where the 
aboral and oral branches unite, 1. e., the base of the fork. But 
the feature of particular interest in this case is the later formation 
of another aboral tentacle opposite the base of this branched ten- 
tacle. ‘his tentacle appears in surface view on the upper left 
