THK AXIAL GRADIENTS IN HYDROZOA. l8l 



hibited forms advance slowly in contact with the bottom, being 

 unable to support themselves free in the water, others revolve 

 with little or no advance and some show reversal in the direction 

 of locomotion (Figs. 6, 8). In these reversal of the susceptibility- 

 gradu-nt and the reduction gradient (Child, '25) has also been 

 observed. 



These modifications represent an extension of basal character- 



- and behavior toward the apical end, i.e., a change of the 

 inon- .tpic.il level- to the condition and behavior of the b 



Is. I :.<: -t degree of such modification con-i-t- merely in 

 an inn. : the basal thickening, an extension of the R v 



immigration tv. ard the apical end and an increa-e in number of 

 immigrating .ell- I i' s r . 4). All gradations occur between this 

 condition and forms in which almost the whole of the blaMula 

 \\all i- involved in immigration and ingrowth . 8, 9) and in 



some cases th( ells of the basal region become so altered that 

 v emigrate in-tead of immigrating (Fig. 6). 

 The tlith n niially inhibited plunuhe which result from earlier 

 lih<- I inures 4-9 are more or less elon-ated fonn> with 

 entoderm, often completely solid. Since they are 

 u-ually not Milih icntly active to swim free, 1-M-i.m, .ti.,n take- 

 pi. iee in i>nta<:t with the bottom, in most Ca-r- \\itli a]iital end 

 in a<l\ainf, but in some reversed, as noted above. A- riliary 

 arti\ it\ <l- es these larva? do not attach by the ori-inal apical 



end, but Dimply come to rest gradually with some meridian i the 



itudinal Mirface in contact and more or le-> Ilatu i ins 



al. >ng i hi- IIM-I idiaii. In the absence of difterential acclimatinn or 



\n\ development may go no further, or one or m< -Ion 



3 nia\ de\elnp slowly. In any case the bn.ly IM 

 eiu loM'd in a drlirate perisarcal secretion. 



/ 'he Mi" c Degrees of Differential Inhibition. These 



appear mme frequently in the higher concentration- or in 

 material subjected to inhibiting action from the beginning of 

 development. They constitute a continuous with the 



tonns already de-cribed and are SC] ! only for coiuenience. 



In material exposed to .-ufticiently high concentrations or degrees 



(tion from i he early cleavage stages, elongation of theblastula 



3 not < iccur, the \\ hole wall remains thick and immigration and 

 mas- in.urouth occur in all regions, resulting in a spherical >olid 



