Feedback Factors Affecting 



Sexual Differentiation 



in Hydra littoralis 



W. F. LooMis 



The Looniis Laboratory, Greenwich, Connecticut 



We have been trying to induce sexual differentiation in Hydra 

 for some years now, because this instance of celhilar differentiation 

 is controlled externally by the water in which these little animals 

 live. This circumstance allows the investigator to analyze samples 

 from cultures that have turned sexual, and then try his hand at 

 recreating such water artificially. In this way, an approach to 

 understanding the biochemical variables that control cellular differ- 

 entiation becomes experimentally possible. 



We have found Hydra to be nearly ideal for such a study. Thus, 

 any desired level of population density within a culture may be 

 maintained indefinitely by simply removing all the baby Hydra 

 that are produced daily by budding, baby Hydra being distin- 

 guished from their parents by the fact that they do not yet possess 

 buds of their own. Secondly, Hydra may be kept in simple saline 

 99% of the time, for they can feed on enough brine shrimp in fifteen 

 minutes to supply their nutritional needs for the ensuing twenty-four 

 hours. All the tedious routines of sterile tissue culture, therefore, 

 become unnecessary when this instance of cellular differentiation 

 is selected for study. Thirdly, the end result of cellular differentia- 

 tion in this system is unusually clear-cut, for even an inexperienced 

 observer can identify functional testes (or ova) on a Hydra if 

 a dissecting microscope is available. Finally, since the differentia- 

 tion of interstitial cells into gonadal tissue is an accessory path- 



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