432 THE BIOLOGY OF HYDRA : 1961 



GROWTH STIMULATING PRINCIPLE IN HYDRA 



It is interesting at this time to consider the factors which stimu- 

 late growth in hydra and which are under the direct control in 

 certain body regions of growth inhibiting principles. In order to 

 demonstrate the existence of a growth principle in hydra it is first 

 necessary to define the action of this principle. It has been previ- 

 ously stated that hydra possesses two "growth" regions in its body, 

 the hypostomal growth region and the budding region. It is well 

 known that if the hypostome is removed from a hydra, a new hy- 

 XDOstome is always formed on the distal portion of the excised body 

 column. The formation of a new hypostome may be interpreted as 

 follows: after the excision of the hypostome, the gastric region is 

 no longer under the influence of the growth inhibiting principle 

 which normally diffuses proximally from the hypostomal region. 

 Therefore, growth substances present in the gastric region are 

 activated and a new growth center is established. On the other 

 hand, if an animal is excised through the gastric region, the 

 proximal portion of the region containing the hypostome al- 

 ways forms a new gastric region, peduncle, and base — never an- 

 other hypostomal region. Again we may say that a growth inhibit- 

 ing principle from the hypostome is inhibiting head formation in the 

 proximal region. 



If the foregoing analysis is correct, it should be possible to 

 initiate head formation in the proximal portion of an excised gas- 

 tric region by supplying additional amounts of the growth principle 

 to this area. Presumably an excess of a growth stimulating princi- 

 ple would overcome the influence of the growth inhibiting prin- 

 ciple. 



In order to demonstrate that a growth principle exists in meta- 

 bolically active regions of the hydra, and that this principle is 

 capable of diffusing from these regions and stimulating cell growth 

 and division in adjacent regions, the following experiments were 

 conducted. 



Two different species of hydra were employed in these experi- 

 ments. One species was the common brown Pelmatohydra oligactis; 

 the other was a new species, Htjdra pirardi, recently discovered in 

 Belgium by Dr. Paul Brien. When these two species are grafted to 



