THE AXIAL GRADIENTS IN HYDROZOA. 



ent levels are less than in the latter. The immigration of the 

 entodermal cells singly and their later arrangement in a layer 

 makes it difficult to believe that this entodermal gradient repre- 

 sents differences present in the cells at the time of, or preceding 

 immigration. If it represents real differences in physiological 

 condition, it is probably superimposed on the entoderm by its re- 

 lation with the ectoderm, the more active regions of the latter 

 determining, perhaps by transmission, greater activity in the ad- 

 joining entoderm. 



As the planula elongates a second region of high susceptibility, 

 reducing po\\-<-r and rate of staining gradually appears at the basal 

 end (Fig. 15) and from this region a gradient, opposite in direc- 

 tion to the original gradient, develops. The stage at which this 

 seo ulient appears varies somewhat in the different species 



and in different cultures of the same species. Under good condi- 

 tions it usually appears somewhat before the planula attains its full 

 length. Under slightly depressing conditions it commonly appears 

 somewhat earlier, but under strongly inhibiting conditions its 

 appearance may be completely inhibited. The portion of the 

 planula body over which the second gradient extends during the 

 vmuning stage varies considerably, being usually one fifth 

 to one third the total length. Undoubtedly the degree of develop- 

 nii ut of this second gradient before attachment depends on the 

 ' at which it fn ->t appears and on the condition of the larva. 



This sc radient is the first indication of the hydranth-stem 



axis. \- the -tage of attachment approaches the activity of the 

 original apical r. -ion decreases and the original apico-basal 

 gradient hec>nies less distinct, while the new gradient at the basal 

 end becomes longer and more distinct. During this period the 

 swimming activity decreases and sooner or later attachment by 

 the original apical end occurs (Fig. 16), a perisarc is secreted and 

 elongation of the basal region to form the hydranth and stem 

 us (Fig. 17). 



Thus far no indications of the original apico-basal gradient 

 have been observed at the stage of Fig. 17. Apparently the only 

 gradient present is the secondary gradient which originated at 

 the original basal end. The objection nay be raised that the pres- 

 ence of the perisarc and the attachment of the original apical end 



