Form Regulation in Harenactis attenuata 355 
In such pieces the extruded mass of mesenteries and muscles can 
readily be cut off (Fig. 2). The operation of removing this mass 
often stimulates the piece to further contraction and extrusion, so 
that a second and sometimes a third mass may be removed. In 
this manner exactly the same result may be accomplished more or 
less completely as by the method of dissection, and much more 
readily. It is usually unnecessary to remove the free borders of 
all mesenteries throughout the whole length of the piece in order 
to produce rings. If the longitudinal wounds are sufficient to 
approximate the oral and aboral cut edges in some degree, they 
usually meet, especially if the length of the piece in its contracted 
condition is much less than its diameter. In such pieces the shape 
alone determines that union of oral and aboral cut surfaces with 
each other rather than the usual method of closure shall occur, 
simply because when the oral and aboral ends of the body-wall 
roll inward after the operation they are not long enough to form a 
closure across the axis in the usual manner and contraction con- 
tinues until oral meets with aboral cut surface and union occurs. 
In general the shorter a piece the more likely it is to form a ring. 
But since in the very short pieces regulation often does not proceed 
beyond the formation of new tissue along the line of union, I have 
found it preferable to use longer pieces and to induce this method 
of closure by the operations above described. 
There is, however, still another very simple method for produc- 
ing rings, one which requires no second operation. ‘This consists 
in the removal of the distal ends of invaginated individuals. This 
method is illustrated by Fig. 3, the broken line indicating the level 
of section. By this operation a short piece is isolated which con- 
tains none of the longitudinal muscles or marginal mesenterial 
organs and at the same time each mesentery has a cut edge through- 
out the length of the piece. Consequently the conditions for the 
formation of rings are established at once in these pieces and as a 
matter of fact such pieces form rings in practically every case, the 
only exceptions to the rule being cases in which other disturbing 
factors are involved. 
The union between oral and aboral cut surfaces in the rings 
occurs in the manner indicated in Fig. 4, a diagrammatic section, 
