THi; AXIAL GRADIENTS IX HYDROZOA. 195 



ami the interior pole, have also undergone reversal, so that instead 

 of immigrating into the blastocoel the cells emigrate. 



It may In- noted in passing that the less extreme degrees of 

 differential inhibition (Figs. 5-9) show interesting resemblances 

 to the < - -'ni l.i- and related forms such as solid blast ul.r ot the 

 ci hinoderm-. which also result from differential inhibition 

 'Mai-Arthur. '24 , In both the hydrozoan and echinoderm. 

 modification- the entoderm and in echinoderms the mes- 

 emliyme 'y increased at the expense of ectoderm. 



! .1 ' .m-e the lo\\er levels of the gradient are normally U 1 -- active 

 an<l therefore less susceptible and less inhibited than the higher 

 Is. Actual reversal of the susceptibility gradient may al-o 

 occtir in both hydrozoan and echinoderm. The es of 

 of cell f. 6) correspond most closely to the cv^ 



'18 has maintained that gastrulation i> due to difference 

 in the degree oi -welling of internal and external part- of the cell- 

 i on, crned and that lithium salts and someoth- it- determine 



exogastrulation by reversing these differences. Whether or not 

 tin- Interpretation be correct, the differential action of the agents 

 -liou - dearly the same relation to the gradient in both hydro/oan 

 and echinoderm. 



That the inhibition, even though it does not obliterate polarr 

 inieilen - with the usual sequence of events in latei i> 



sho\\ M by the fact that when acclimation or rci.\cr\ occurs, the 

 ouurouth may be apical (Fig. 16) or both apical and ba-al (1 

 17 . that i-, the stolon may arise from the primary gradient or 

 both primary and secondary gradient and probablv aln> 

 the -econdary alone (Fig. 19). These differences un- 

 doubtedly (U-|)end on differences in stage of development at time 

 ot e\|o-ine and on degree of inhibition. The development of 

 bi-ili apical and basal stolons indicates that under the inhibiting 

 condition- the two ends have become independent of each other. 



The conditions determining the "adventitioti-" polariiie- in 

 the acclimation and recovery' of apolar form -. 24 joi an- 



ob\ ion-l\ accidental for no two individuals are alike a- nvard- 

 ili/ation and time of appearance of the different axes. K\ i- 

 dentl>- the-e axes originate as local areas of -li-htly greater 

 acti\it\" in -oine region or region- of the mass, .^onie of them 

 max icpie-ciu |)er-i-!in^ traces of the original polarit\ - or 



