DIFFERENTIAL INHIBITION IN OBELIA 477 



of the hydrotheca ; its shape is roughly ovoid. No sign of 

 tentacles whatsoever (Text-figs. 1, d, and 3, a, and PI. 26, 

 fig. 4). 



Fourth Stage. Sometimes the distal portion of the zooid 

 may become separated from the rest (Text-fig. 4). The form- 

 determining properties of the zooid have become less powerful 

 than the surface tension acting upon it, and it has accordingly 

 become spherical, nowhere touching the hydrotheca and 

 connected by a thin stalk to the hydrocaulus. At this stage 

 and later the flow in the tube is irregular. It appears to be 

 maintained by pulsations of the stolon (see p. 479) (Text- 

 fig. 1, e, and PI. 26, fig. 5). 



Fifth Stage. The process has been continued until the zooid 

 is represented only by a tiny knob (often containing pigment) 

 smaller in diameter than the hydrocaulus (Text-figs. 1, /, 

 and 5). Occasionally the process is carried further and the 

 hydrotheca becomes empty. This only occurs a considerable 

 time after stage 5, and is mainly a mere degeneration effect. 



It should be noted that in those cases where the colonies 

 contained gonothecae, medusae were not liberated if resorption 

 had started. During resorption the zooids are perfectly 

 healthy and transparent. Dead tissues can always be distin- 

 guished (opacity, &c.). Small masses of dense pigment are 

 often found in the partly resorbed zooid, representing products 

 of degeneration. 



As resorption goes on, the material derived from the zooid 

 passes into the hydrocaulus, and from the proximal (cut) end 

 of the latter a stolon begins to grow (Text-fig. 3, a and c). 

 It is very transparent and clear, and may grow to the length 

 of 10 mm. or more, affixing itself to the substratum. It some- 

 times happens that a small portion of what is left of the zooid 

 in stages 3-5 is completely nipped off (by surface tension) 

 from the hydrocaulus. It remains in the hydrotheca and dies 

 (Text-fig. 4). 



In the earlier experiments the solutions used were too strong, 

 but even so a differential effect was obtained (Table I). 



