PHYSIOLOGICAL CIL^RACTERISTICS OF AXIATE PATTERNS 97 



ula. The same gradient relations have been found during development of 

 two other calyptoblast species. The planula of a Japanese species, Ser- 

 tularella minurensis , develops the secondary gradient in the original basal 

 region at a somewhat earlier stage; in the fully developed planula the 

 end originally basal is already more susceptible than the apical end, and 

 the secondary gradient has extended over most of the body length. 



/-: .-*^ 



D 



Fig. 29, A~E. — Susceptibility gradients in development of the hydrozoan Pkialidinm. A, 

 oocyte isolated by teasing; B, first cleavage (in A and B disintegrated portion indicated in 

 broken line); C, early planula with gradient from apical end; D, later planula with second 

 gradient at original basal end; E, after attachment by original apical end; secondary gradient 

 of basal end becomes hydranth-stem axis. Arrows show direction of progress of disintegration 

 (from Child, 1925a, with modifications). 



The egg of the gymnoblast hydroid Corymorpha palm a, when shed, 

 sinks and adheres firmly to any solid substratum. A gradient is present 

 in early stages with high end at the free pole, but it has not been possible 

 to determine whether the egg orients itself in sinking or whether the gradi- 

 ent results from the differential in conditions from the free pole to the pole 

 in contact. The planula never swims but may progress a short distance in 

 contact with the substrate with apical end in advance and secreting peri- 

 sarc as it goes. In the early planula differential susceptibility, permanga- 

 nate reduction, methylene blue reduction, and the indophenol reaction 



