414 W. E. RITTER AND M. E. JOHNSON 
ATTEMPT TO CONNECT THE FORMATION OF WHEELS WITH MOR- 
PHOLOGICAL, PHYSIOLOGICAL, AND MECHANICAL PHENOMENA 
PRESENTED BY THE ANIMALS 
1. Segmentation of stolon and the deploying point 
To find other factors entering into the wheel production, a study 
of the structure of the chain was made. The portion of the chain 
in which the zooids are in single file is of the same general form as 
that of other species. The incipient zooids, marked off by the 
infolding ectoderm, have their aboral ends uppermost, and the 
dorsal side of each against the ventral side of its neighbor, the 
dorsal sides being towards the base of the stolon. The blood sup- 
ply passes out through one-half of the large axial blood vessel and 
back through the other half. The segmentation of the stolon in 
some cases extends to the root of the stolon, in others not quite so 
far. <A possible significance of this variation will be pointed out 
in another connection. Where segmentation extends to the root 
of the stolon, the more proximal segmentation lines are very 
irregular and this fact may be of considerable interest in a way we 
shall not stop to consider here. Judging by some chains, one 
would say that the segmentation begins at the sides of the stolon, 
but others lead us to suppose the beginning is above and below 
(along the genital rod and the neural tube) while in still others 
it seems to be equally advanced in all parts of the circumference. 
The latter condition is probably the usual one. 
2. Position and relation of the zooids in the chain from the 
deploying point to the twist 
a. Shifting of the zooids. What we call the deploying point, is 
the point, or region where the zooids, by moving alternately to 
the right and left shift from single to double file. While the zooid 
is moving out, it also moves upward and begins to turn, so that 
its dorsal side faces out instead of toward the base of the stolon. 
These changes ,take place gradually. The oral ends shove out 
and begin to turn and before the turn is complete, the aboral ends 
