W. F. LOOMIS 347 



were also necessary (19). When these other factors have been 

 identified, and their appropriate dosage determined, it should be 

 possible to add the necessary components to the reservoirs of 

 Figure 4 and have the constantly-washed Hydra in the syphoned- 

 beakers differentiate sexually because they "think" they are 

 crowded, i.e. their ectoderm being exposed to the same conditions 

 found within the halo zones of a crowded culture. 



Since the present multi-factor approach has evolved gradually 

 over several years, it may be worthwhile to review briefly the route 

 by which this investigation has progressed since this provides a 

 framework within which to discuss various important observations. 



1) po, 



Looking back, even our earliest observations suggested that 

 sexual difl^erentiation occurred under conditions of partial anaero- 

 biosis (11). Thus, we found that 1) a score of Hydra turned sexual 

 in a stagnant aquarium tank full of living Daphnia; 2) they 

 reverted to the asexual state a few days after the aerator of the 

 aquarium was turned on; 3) the shape of the container, and its 

 surface/volume ratio, strongly influenced the reaction as seen in 

 Table 2; 4) crowding Hydra almost automatically induced them 

 to turn sexual in BVC while 5) stagnation constituted a reciprocal 



TABLE 2 



Percentage of sexual forms and oxygen tension in cultures of differing 

 surface/volume ratio. 



Each culture consisted of 25 Hydra in 25 ml. BVC solution contained in a 50 ml 

 beaker and three sizes of Petri dishes. 



