Form Regulation in Harenactis attenuata 381 
and that, because of the character of the material, it must approach 
more or less closely to the usual course of development, according 
to the conditions. 
In the rings various factors prevent the formation of complete 
animals and, as has been shown, these factors operate in different 
degree in different cases. The number of tentacles in the group 
is undoubtedly determined by the number of mesenteries. In 
cases where no new mesenteries are formed (e. g., Figs. 8, 10, 17) 
the number of tentacles is determined simply by the number of 
mesenteries involved in the region of localized growth (Figs. 18 
to 20.) In cases such as No. IX (Fig. 29) where new mesenteries 
are undoubtedly formed, the number of mesenteries and tentacles 
is undoubtedly determined by the distance factor in correlation, 
1.e., by the distance within which the formation of one mesentery | 
or a pair inhibits the formation of another. 
4. Localization of the Groups in the Oral-Aboral Direction 
In the above description of the various cases they were divided 
into two series, those in which the groups formed on the line of 
union with some of the tentacles arising from the original aboral 
end of the body-wall and from the line of union itself (Cases [ to V, 
Figs. 6 to 20), and those in which they were situated wholly on the 
oral side of the line of union (Cases VI to [X, Figs. 21 to 30). 
As regards the conditions determining these two different locali- 
zations, my records indicate that the pieces in the second series 
were those from which the mesenteries had been more completely 
removed, at least near the oral end, than in the pieces which form 
the first series. In most of the pieces in the second seriesthem sen- 
teries were extruded and removed more than once, while in the 
first series only one extrusion and removal occurred in most cases. 
Referring to Fig. 31 in which the result of the removal of extruded 
mesenteries is indicated as it would appear if the piece could 
resume its original condition of distension, the condition in the 
first series is something like that represented by the removai of 
parts above the line a, while that in the second series is represented 
by removal of parts above the line . Mesenteries were extruded 
