426 THE BIOLOGY OF HYDRA : 1961 



that the concentration of food reserves ( glycogen, neutral fats, 

 protein reserve droplets) along the body column is directly in 

 proportion to the metabolic activity of a particular region. Areas 

 of low metabolic activity contain few gastrodermal inclusions while 

 areas which are capable of reducing methylene blue when applied 

 to the living animal contain an excess of food reserves. Further- 

 more, a histological examination of different body regions in 

 hydra has revealed that interstitial cells and gland cells are scarce 

 or lacking altogether in regions of low metabolic activity and are 

 abundant in regions of high activity. 



A question which immediately comes to mind is reminiscent 

 of the old "hen and egg" question, i.e. do regions of high metabolic 

 activity possess this activity because of the presence of food in- 

 clusions and interstitial cells in this area, or do these areas contain 

 these specific cells and food inclusions because of the general 

 active metabolism of this area which is not directly related either 

 to interstitial cells or specific food inclusions? 



Nearly all workers in the field of metabolic gradient have 

 stressed the fact that the "head" region (hypostome region), be- 

 cause of its high metabolic activity, in some manner supresses the 

 formation of another head in its immediate vicinity. For this 

 reason, a bud never forms directly beneath the head of the parent 

 under normal conditions, but begins as an outpushing of the body 

 roughly midway between the hypostome and basal disk. Such a 

 hypothesis immediately suggests that the "head" region liberates 

 an inhibitive substance which in some manner prevents the cells 

 of the adjacent gastric region from entering the active cell divisions 

 which would eventually lead to the formation of a bud. If this is 

 indeed true, then it is first necessary to demonstrate that such an 

 inhibitive principle exists, and secondly, if it does exist, it is 

 necessary to determine why it doesn't affect cellular divisions in 

 the head region itself. 



Another series of questions are closely linked with this same 

 problem. Brien ( 1 ) has shown that the sub-hypostomal region 

 of hydra is an active growth center. Constant cell proliferation 

 in this center forces cells distally towards the tentacle tips and 

 proximally towards the basal disk where these "migrating" cells 

 atrophy and are sloughed off the body column. Thus, by constant 



