434 THE BIOLOGY OF HYDRA : 1961 



3 striking instances H. pirardi portions began tentacle formation 

 before the H. oligactis portions had detached. In these 3 cases 

 the results were most interesting. Small tentacles began to form 

 from the peduncles of F. oligactis a day after tentacle formation. 

 had begun on H. pirardi. When the grafts were fed with brine 

 shrimp, it was noticed that the bulk of ingested food materials were 

 taken into the digestive cells of the peduncle of F. oligactis. Such a 

 phenomenon never occurs under normal conditions. 



Three days after feeding the 3 animals were sectioned for his- 

 tological study. It was found that the normally wasted peduncular 

 digestive cells of F. oligactis were full of protein reserve droplets. 

 Interstitial cells had invaded this area and appeared in concentra- 

 tions comparable to the normal growth region of the hydra. Sev- 

 eral dozen interstitial cells had transformed into gland and mucous 

 cells, and in the lower regions of the peduncle where a basal disk 

 would normally be expected to form, a new hypostome was nearly 

 completely elaborated. 



Thus, it appears that F. oligactis does contain specific growth 

 stimulating principles within its tissues. When these principles (or 

 principle) are present in a body region in sufficient concentrations, 

 this region will take in large amounts of food after each feeding and 

 will subsequently be invaded by interstitial cells. It is hypothesized 

 that the metabolic activity of a given region of hydra is dependent 

 upon the amount of growth stimulating principle which is present 

 and which is not being affected l)y a growth inhiliiting factor. 



Unfortunately little is known at the present time concerning the 

 nature of either the growth stimulating or inhibitive principle. A 

 method has recently been devised whereby it is possible to collect 

 the stimulating principle in agar blocks and introduce it into any 

 desired body region of another hydra. Burnett and Schwager are in 

 the process of elucidating the chemical nature of this principle. 



The growth inhibiting principle has not been isolated from the 

 tissues of the hydra at the present time. It will be interesting to de- 

 termine whether the inhibitive principle which acts in the tissues 

 of the hydra by diffusing proximally from the growth region is the 

 same as that found by Lenhoff and Loomis (13) which can limit 

 the asexual reproductive process of a colony of H. littoralis. The 

 inhibitive principle described by Lenhoff and Loomis is heat- 



