I04 PATTERNS AND PROBLEMS OF DEVELOPMENT 



reduction of permanganate in the body of hydra, in tentacles, hydranths, 

 stems, and stolons of hydroids, in medusa buds and fully developed medu- 

 sae parallel closely those observed by other methods. They parallel the 

 electric-potential gradients, but apical regions which have been found to 

 be externally electropositive and those electronegative to other regions ap- 

 pear as regions of high susceptibility and high rate of permanganate re- 

 duction. Assuming that the data on potential differences are correct and 

 that the direction of sign change may result from different kinds of physi- 

 ological activity in different species, particularly in reconstitution, this 

 lack of correspondence as regards direction of gradient is to be expected, 

 since differential susceptibiUty and differential reduction apparently indi- 

 cate merely differences in rate or intensity." 



It is a point of particular interest that early stages in the form of buds 

 of hydranths, tentacles, medusae, and stolons appear as local gradient sys- 

 tems, at first more or less radial from a center and becoming longitudinal 

 as outgrowth proceeds. In the naked contractile hydra, however, the high 

 end of the bud gradient is not always continuously apical. Both the reduc- 

 tion and the susceptibility gradient of the bud may be reversed by con- 

 tractile activity of the parent body, which often involves the basal region 

 of the bud. Contractile activity of the stalk in detached animals increases 

 its susceptibility, and bending of the hydra body may increase suscepti- 

 bility of the contracted side ; these effects parallel those observed in dye 

 reduction (p. loi). 



In branching hydroids with more or less definite growth form of the 

 whole {Pennaria, Obelia, Gonothyraea) the multiaxiate system shows a 

 basipetal decrease in susceptibility similar to that in multiaxiate algae 

 (p. 86). In these cases susceptibility of the terminal hydranths decreases 

 from the apical hydranth of the system basipetally and also basipetally 

 in each branch, but with occasional irregularities. This gradient is in the 

 same direction as the gradient in rate of hydranth reconstitution observed 

 in another species of Pennaria (pp. 38, 39). 



The gradients in a monophyid siphonophore are of interest. The coeno- 

 some or stolon arises from one side in the median plane of the bilaterally 

 symmetrical nectomedusoid. Groups of zooids, each consisting of a nu- 

 tritive zooid or manubrium with tentacles developing near its base, a me- 

 dusoid, and a bract, develop successively from the region of origin of the 



" For data on differential susceptibility and permanganate reduction see Child and Hyman, 

 1919; J. W. MacArthur, 192 1 ; Wcimer, 1928, hydra, several species; Child, 1926a, Corymorpha; 

 Child, 1919a, d, i()2id; Uyman, 19206, various hydroids and medusae. 



