430 THE BIOLOGY OF HYDRA : 1961 



liberates an inhibitive principle similar to that liberated from the 

 head region of the parent. 



Lenlioff ( 12 ) has demonstrated that, if the gastric region of a 

 H. littoralis containing a bud is transected immediately above that 

 bud, head formation of the parent is inhibited at the site of section 

 even after the bud separates from the parent stalk. This observation 

 strongly indicates that an inhibitive substance is released from the 

 tissues of the bud. However, the answer is not as simple as that. 

 If it is true that the inhibitive substance l^ecomes more dilute as it 

 diffuses along the column, the regeneration of the parent head 

 should not be affected if the head is transected at the level of the 

 sub-hypostomal growth region rather than through the gastric 

 region as demonstrated by Lenhoff. 



In order to test this hypothesis, parent hydra, bearing buds 

 in all stages of development were transected either through the 

 middle of the gastric region or through the sub-hypostomal growth 

 region. Subsequent examination of these animals revealed that ani- 

 mals which had been excised through the growth region regener- 

 ated within twenty-four hours and were not affected by the presence 

 of a bud on their column. However, animals which had been ex- 

 cised through the gastric region were inhibited in their regenerative 

 processes and had not even begun tentacle formation after twenty- 

 four hours. Interestingly enough, parent animals, excised through 

 the gastric region and containing buds which had formed peduncles 

 had begun tentacle formation after twenty-four hours. It will be re- 

 membered that the peduncular region is capable of inhibiting in 

 some manner the passage of a growth inhibiting principle. 



These results indicate that a developing bud does contain a 

 growth inhibiting principle, and that this principle in some way 

 is gradually rendered impotent as it diffuses from the bud along 

 the length of the parent column. 



Many aspects of hydra's biology can be tentatively explained 

 in light of a growth inhibitive substance. For instance, many specu- 

 lations have been made to explain why a hydra containing several 

 buds always produces them successively, and why the buds, in ad- 

 dition to the fact that they are in different stages of development, 

 are arranged in a helical pattern and are on essentially different 

 sides of the stalk from one another. 



