THE AXIAL GRADIENTS IN HYDROZoA . 183 



and the pattern is no longer axiate, but merely surface-interior 

 (Child, '24, pp. 57, 93). 



I >ll I I.KI-.M IAI. A i CLIMATION AND DlFFKKI.M 1 \! Rl>''\KRY. 



I "nder "normal" conditions the blastula develop- into the 

 elongated free-swimming planula (Fig. 14,). which -ho\\~ .it first 

 .in apico-ba-al gradient, but later develop- at the basal end a 

 second gradient opposite in direction to the first, as dc-cribed in 

 the prc< -eding paper (Child, '25). Later the planula attaches 

 t by the original apical end and the hydranth--tcni a\i- 

 devclop- f'n.in the original basal end, i.e.. from the -ccondary 

 gradiei is). As pointed out in th< paper, the 



!)\dr.inth--teni a\i- represents a new polarity ari-in^ by a pn 

 of bnddiiij I'ruiM the original basal end of the plamila. 



In the li.\\er concentrations of agents which produce at lir-t 

 some di of differential inhibition withe. in cunipleteK- cblit-r- 



atinv pnl.iriiy, differential acclimation or acquirement "f ti-K-ram-e 

 may ^radiialK ccur in the course of a fe\\ da\- \\ith further 

 iii'.ditiratiun of development. Similarly ditterenti.il recovery 

 m.i\ itcciir '.ii return to well aerated sea water after tcniiM>: 

 e\p(.-uie ti> concentrations which produce differential inhibition. 

 Within a certain range of differential inhibition the regions nu>-t 

 inhibited acclimate or recover most rapidly or IDM-I < i.mpleteK , 

 i- indicated by further development. Since differential 

 acclimation and differential recovery are -eci.ndaiA cha- 

 \\lncli t.ike place- following a differential inhibiting action, it i> 

 e\ ident that they can modify only the later stages of development. 



The tir-i indication of differential acclimation or differential 



ivery in differentially inhibited planul.e uhicli -till retain 



some d( -f polarity is the outgrowth of .1 -tolon from one or 



both end>. 1 iv:. 16, apical stolon, and Fi.u- 17. apic.il and ba-al 



>tol<.n-, as een from above, show characteristic form-. In the 



of 1 . ts the larva probably came to rest with the body bent 



upon it-elf and gave rise to a stolon from each end, thou-h it i> 



-ible thai one or both of these stolon- repre-ent ne\\ ]io].;rii ic-. 



In the-e and later figures the heavy line indicate- peri-arc, the 



liijit line the niti-r surface of the coenosarc. 



\- I have pointed out in an earlier paper (Child. '231. hydioid 

 stolons repre-ent -omewhat inhibited physiological gradient- or 



