236 



THE BIOLOGY OF HYDRA : 1961 



where normal absorption could take place. A diagrammatic pic- 

 ture of the apparatus which was designed to inject a measured 

 volume into the individual organism is shown in Figure 1. The 

 apparatus features a micrometer-driven micro-liter syringe for de- 

 livering quantities in the micro-liter range. 



^) ^ fe 



to foot switch 

 for motor 



to foot pedal 

 for release bar 



Fig. 1. Micro-injector for feeding Hyc/ro. 



In our standard injection test, adult Hydra without buds were 

 selected from the cultures maintained on heated Arteniia. The 

 animals were placed in 9-depression spot plates in large Petri 

 dishes, and each one was force-fed 0.2 [xl from a glass capillary 

 containing semi-solid agar in which the experimental diet was 

 dissolved or suspended. Twenty-four hours after injection, the ani- 

 mals were examined under a dissecting microscope, and the num- 

 ber of new buds in each dish of nine Hydra was recorded and 

 compared with that of the unfed control dish. 



The relation of growth response to the quantity of material 

 injected is shown in Figure 2. The response was proportional to the 



