292 Experimental Zoology 
the united parts continue to change their form, in the same way 
as do separate pieces, until the relation of the parts of the cut 
surfaces is the same as that for the normal in this region, when 
further adjustment ceases. In the “long” hydra the same kind 
of changes take place, but the only way in which the typical 
relations could be adjusted would be either by absorbing the 
excessive parts, or by each piece, acting independently, assuming 
the typical form. The latter method is the one followed. This 
attempt to account for the different behavior of the two grafts 
may appear to be little more than a restatement of the facts; 
but if the restatement is correct, it has at least the advantage 
of referring the results to the factors that are operating. The 
hypothesis also involves a principle that will account for the 
other formative changes to be described. 
Tf two pieces of hydra are united to each other by their anterior 
cut surfaces (Fig. 5), tentacles soon appear around one or both 
pieces near the line of union. If each half develops tentacles, 
the halves may pinch apart in the intermediate region (Fig. 6); 
if only one set of tentacles develops, a mouth may form, and the 
two pieces have a single crown of tentacles. Little by little the 
two pieces fuse together lengthwise into a single body as shown 
in Figs. 7 and 8, until finally one hydra results. The develop- 
ment of the tentacles at the line of union may possibly be ac- 
counted for by the inability of each piece to adjust its tension 
relations to the other; for although when halves are united in 
opposite directions the tension at the place of contact is the same 
as in the normal, since like regions are united, the reversal of the 
direction of the tensions in the opposed pieces makes it impossi- 
ble for further relations to become established except by each 
part behaving as a separate individual. Here we meet with the 
conception of polarity as involved in the pressure relations. The 
polarity from this point of view is an expression of the graded 
pressure relations from one end of an organism to the other, 
which in turn may be an expression of the gradation of the 
tissues, and in turn may itself, under certain conditions, be 
the cause of the differentiation. 
