I70 PATTERNS AND PROBLEMS OF DEVELOPMENT 



move along the bottom of the container and finally come to rest with 

 one side, instead of the apical end, in contact. After several days indica- 

 tions of differential conditioning, or of differential recovery if they are 

 returned to water, appear; these consist in outgrowth of stolons from the 

 apical end or from both ends (Fig. 58, B, C), representing either the 

 primary gradient of the planula or both gradients (pp. 96-97). They 

 may continue to grow as stolons at the expense of other parts of the 

 planula body or, if returned to water, may recover suf]ficiently to give 

 rise to a hydranth-stem axis (Fig. 58, D). Failure of the planulae to 

 attach by the apical end and development of stolons instead of hydranth- 

 stem axes from one or both ends occur with very slight degrees of inhibi- 

 tion — for example, in slightly crowded cultures or with insufficient aera- 

 tion, as well as with low concentrations of various chemical agents. 



The normal development of the gymnoblast hydroid Corymorpha after 

 attachment of the planula is shown in Figure 59, A-C. As indicated in 

 Figure 30 (p. 98), the high end of the primary gradient is apical, and a 

 secondary gradient appears basally in the course of development. The 

 egg of Corymorpha is so opaque that effects of differential inhibition on 

 early development are obscured; but polarity can apparently be obliter- 

 ated, as in Phialidium, by cyanide, lithium, alcohol, ether, and probably 

 by methylene blue and neutral red, the embryo remaining spherical within 

 the membrane, though alive. With less extreme inhibition, retarding de- 

 velopment but not preventing hatching and not obliterating polarity, the 

 planulae become much longer than normal because the less susceptible 

 basal and middle regions are less inhibited than the apical end (Fig. 59, 

 D, E). Further development of these forms in the inhibiting agent is 

 indicated in Figure 59, F-H. If tentacles appear at all, they do not de- 

 velop beyond early stages; distal tentacles are more inhibited than proxi- 

 mal and often do not form (Fig. 59, F) ; often development does not go 

 beyond the stages of Figure 59, D and E. In all cases the hydranth region 

 is small and the stem large and elongated. Evidently the less susceptible 

 stem has been able to grow to a greater extent, as compared with the 

 hydranth, than in normal development. If the forms of Figure 59, F-H, 

 are returned to water, hydranth development proceeds, the hydranth 

 growing at the expense of the stem, so that the usual proportions are 

 approached, that is, differential recovery occurs. With slightly greater 

 inhibition development rarely goes beyond the stages of Figure 59, / and 

 J , except after return to water. 



Differential regression of development may also be brought about by 



