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. 



If 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 Hne 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. 



