272 C. M. Child 
oral side only of the line of union, of more or less complete, usually 
radially symmetrical groups of tentacles which undoubtedly repre- 
sent more or less completely oral ends, In the following para- 
graphs some of the probable or possible factors involved in the 
localization and development of these structures are considered. 
2 Localization of the Tentacle Groups Along the Line of Union 
The localization of the groups is one of the most striking fea- 
tures of the rings, and the question at once arises as to what fac- 
tors determine that isolated groups of tentacles, indefinite in num- 
ber and position, shall be formed rather than a continuous series 
of tentacles about the oral end. If the oral end can produce 
tentacles at all why does it not produce them in the usual manner ? 
Manifestly the answer to this question is to be found in some of the 
conditions of the experiment. ‘The characteristic feature of all 
these cases is the union of oral and aboral ends about the whole 
circumference; no free end or terminal region like that from which 
oral structures usually arise, exists in these pieces. It seems 
highly probable that the union of oral with aboral end brings about 
changes in the physiological conditions characteristic of these 
two regions, since the two ends are in direct organic continuity 
with each other. If such changes occur they must consist in a 
decrease in the differences between the two ends and it is probable 
that the new tissue at the line of union is neither oral nor aboral 
in its physiological characteristics, or at least that it is less “oral” 
in character than the new tissue formed by the closure of the oral 
end in the usual manner. ‘The fact that tentacles do not appear 
at all in the rings until long after their formation in pieces of the 
usual sort from the same region indicates that some change, appar- 
ently quantitative, has occurred, for so far as morphological con- 
ditions are concerned there is no obstacle to the formation of ten- 
tacles. Under the usual conditions, 1. e., at the oral end of a 
cylindrical piece the tentacles arise, not directly from the cut sur- 
face, but are localized regions of growth and differentiation in a 
continuous sheet of tissue formed by the closure of the cut ends. 
The continuous sheet of tissue is present in the rings after closure 
and there is no apparent reason why a continuous circle of ten- 
