428 THE BIOLOGY OF HYDRA : 1961 



tions will be described in detail in this paper. A long monograph 

 describing the growth processes exhibited by hydra is now in 

 press (/. Exp. ZooL). This monograph will describe in detail all 

 of the experiments listed in the present paper. 



THE GROWTH INHIBITING PRINCIPLE IN HYDRA 



In order to demonstrate that hydra possesses a growth inhibit- 

 ing principle in its tissues, and that this principle is produced in the 

 hypostomal region, a simple experiment was conducted. It was as- 

 sumed that if an inhibitive principle was produced in the hypo- 

 stomal region and that if this principle diffused in some manner 

 down the body column, it would be possible to induce head forma- 

 tion in the gastric region of the animal by simply preventing the 

 flow of this principle. Such a manuever was accomplished by sim- 

 ply grafting the peduncle of one hydra between the hypostomal 

 and gastric region of another animal. Burnett (3) has shown that 

 the peduncular digestive cells are highly vacuolated and have a 

 wasted, aged appearance. It was thought that perhaps these vacu- 

 olated cells might in some way impede the passage of a growth in- 

 hibitive principle proximally. 



Thirty-nine grafts similar to that described in the preceding para- 

 graph were performed. In 19 cases the hypostomal region plus the 

 transplanted peduncle split from the gastric region and the original 

 peduncle. After the split the gastric region grew a new "head" on 

 its distal surface. However, in 20 cases, after a period of 1-2 days, 

 tentacles formed in the gastric region just below the site of the 

 transplanted peduncle. Eventually an entire hypostome was 

 formed proximal to the grafted peduncle, and the outcome of this 

 experiment was the formation of two completely normal hydra. 



This experiment clearly indicates that cells in the gastric region, 

 when removed from the direct influence of the hypostomal region, 

 are stimulated to form a new "head" region. It may be possible 

 that a growth inhibitive substance produced in the hypostomal 

 region diffuses down the body column inhibiting substances which 

 would normally promote growth in the gastric cells. Moreover, the 

 inhibitive principle must pass down the column by diffusion from 



