ALLISON L. BURNETT 433 



one another there is no celhilar exchange whatsoever between the 

 species, except that the nematocysts of one species are able to be 

 incorporated into the tissues of the other species. 



Twenty Pelmatohydra oligactis were excised through the mid- 

 dle of the gastric region, and the distal excised portions which con- 

 tained the head region were grafted to the growth regions of a sim- 

 ilar number of Hydra pirardi whose hypostomes and tentacles had 

 been excised. After a period of 2-3 days the distal regions of the 

 H. pirardi portions began to form new tentacles and hypostomes. 

 The following day, tentacle growth invariably began on the proxi- 

 mal region of the H. oligactis portions. Ultimately, new head 

 regions were formed on either side of the junction of the grafted por- 

 tions and the grafted animals separated from one another. A true 

 reversal of polarity had thus been effected in P. oligactis, and this 

 had been accomplished without any exchange of cellular material 

 from H. pirardi with the possible exception of cnidoblast cells which 

 would presumably not be directly involved in growth processes. 



These results indicate that a growth stimulating principle is pres- 

 ent in the hypostomal region of H. pirardi and that this principle is 

 capable of passing into the tissues of Pelmatohydra oligactis and 

 stimulating head formation. 



A further series of experiments were conducted to further con- 

 firm the presence of the potential of the growth stimulating princi- 

 ple. It will be recalled that cells of the peduncular region of hydra 

 are highly vacuolated, contain few food inclusions, are metabolically 

 inactive, and are destined to die and be sloughed off the basal 

 disk. Furthermore, the epithelio-muscular cells of his region con- 

 tain very little cytoplasmic RNA, and this area is characterized 

 by the fact that it contains few or no interstitial cells and no glan- 

 dular cells in its proximal regions. 



Thus, it was desirable to determine whether a growth stimulat- 

 ing principle, after being introduced into the peduncular region, 

 would be capable of "rejuvenating" this senescent region. 



Fifty peduncular regions of Pelmatohydra oligactis were grafted 

 to the growth regions of a similar number of Hydra pirardi as in 

 the previous experiment. In 47 cases the peduncles of the P. oligactis 

 portions formed a basal disk and separated from the H. pirardi 

 portions before the latter had begun head formation. However, in 



