THE AXIAL GRADIENTS IN HYDROZOA. ^ 



opposite end of the polar axis from the original dominant region. 

 In that respect it is similar to the " axial heteromorphoses " which 

 are characteristic features of the normal budding and of the re- 

 constitutinn of stem pieces in Tubnlaria and many other hydroids 

 ("Child, '07 :id literature there cited; '15, pp. 91, 92, 133-37). 



The term ' heteromorphosis " is merely descriptive and leaves 

 us without any interpretation of the phenomena concerned. So 

 far as analy/ed physiologically, all cases of so-called axial hetero- 

 morph<>-is are essentially cases of budding which involve the 

 iri.m'n <>f a new polar gradient. Actually every hydroid bud and 



ry luiil in a multiaxial plant is as truly a heteromorphosis as 

 are tin <-s of buds arising at the basal end of an axis. All 



i laid- -ent the determination of new physiological axes 



and tin- r 'iiditiuns, external and internal, determine in each case 

 win-re tin IM-W axes shall appear. 



In xMinc of the tubularian hydroids, e.g., Corymorpha (Torrey 



and m\ own observations), the hydranth-stem axis arises from 

 tin- original aj-ual end of the planula and although the secondary 



lient appears sooner or later at the basal end, physiological 



isolation is not complete and the secondary gradient gives rise, 



at lea-t in its earlier stages only to stolon axes, which are inhibited 



i "hild. '2341 ). In these forms, then, the primary gradient 



the gradient of the hydranth-stem axis. 



SUMMARY. 



: lie axial gradient and its changes during development 



were demonstrated in riiialidium from oocyte to hydroid stage 



by differential susceptibility to various agents, differential reduc- 



if KMnO 4 and methylene blue and differential vital staining. 



elopniental stages of Gonothyraa and Stomotoca served as 



(.-i imperative material. The results of the different methods agree 



ami 'ii^ thrni-clves and with the observed facts of development. 



2. In I'ltiiilidiuin and Stomotoca the polarity of the oocyte is 

 indicated by a gradient with high end at the free pole, low end at 

 the attached pole. This gradient is directly related as regards 

 direction to the differential exposure of the oocyte in the gonad 

 and it is -n L; -r-;ted that the polarity of the oocyte is determined 

 bv this differential exposure. 



