THI. AXIAL GRADIENTS IN HYDROZOA. 179 



DIFI I;KI.\ IIM. INHIBITION IN THE DEVELOPMI \i OF 



THE PLANUI A. 



It i> important to note first of all that there is no evidence of 

 any -pccilic effect on development of any agent used. The 

 relation, ot the differential modifications to the gradient are 

 -imilar for all agents, though the decree of difh-rfnti.il action 

 differ- for dim-rent concentrations and for different a^ent-, bcinv. 

 itest for l.i< 1. i.e., in LiCl the high end of .1 v.radient i> 

 relatively more, the low end relatively le-s inhibited than with 

 oilur agents used. The high degree of differential action of 

 lithium -ah- ha- also been noted by Mac. \rthur Ji in the 

 modification of e< hinoderm development. 



s in<e tin- modifications produced by tin- different a^ent- 

 Minilar, detailed -tatcments of the results obtained \\ith earh 

 it uonld iiiM.Ue much rejK'tition and are omitted for the 

 -ake of bn-\ it\ . The various degrees of modilu atimi an- brietK 

 ribed and data concerning their occurrem < \\ith diitncnt 

 ( om-entratii>ii- .ind agents are given. 



I f Differential Inhibition.- In <>IK entrati.>n- 



-i 'iiu-\\li.it ab>\e those used for developmental modification the 

 cle.; 11 separate into single bla-tomere- r -nll.iie 



into inc-ular rell masses which sooner or later <lie. but in the 

 concentrations n-ed for modification of later -t.i^e-, dea\ 

 proceeds without appreciable alteration. Susceptibility i- fountl 

 to in. rease dm in- early development and the- louci -n-i ept ibility 

 I lea is compared with later sta^e-. to-ctlu-r \\ith 



the I'art that more or less time is neces- r the inhibiting 



a.tion to bet.. me evident, undoubtedly account for the ab-eiue 

 oi modilu MI ion of cleavage with the concentration-, u-i-d. 



I iv;-. I . J and .^ are diagrammatic outline- of bla-tula. immixra- 

 i ion -t.r^e. and i-arly pl.mula as they appi-ar under -^-.od < , ni- 

 di t ion- in the labor.it(r>'. and represent approximately "normal " 

 dexelopmeni. I i'^-. 4-9 show the less extreme d< of dif- 



ferential inhibition. These modification- .ire in general mon- 

 frenuent in material exposed to the lower concent rat ion- from the 

 of <K-\ i-lopment or to the higher concentration- from 

 aiKanced cleavage or early blastula. Thr\ con-i>t in dillerent 

 "I ir md extension of the ba-al thickening of the 



