426 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



nated. It gradually comes to rest, apparently in any position, there being 

 no evidence of orientation when it is rolled about passively. After coming 

 to rest, it flattens to a more or less hemispherical mass and secretes peri- 

 sarc about itself (Fig. 144, A). With further recovery a hydranth-stem 

 axis may develop from its upper surface (Fig. 144, B) ; or two, three, or 

 even four new axes, one a hydranth-stem axis, the others stolons (Fig. 144, 

 C), or all stolon axes (Fig. 144, D) may develop. These stolon axes often 

 give rise later to hydranth-stem axes from their upper surfaces (Child, 

 19256). The contact-free-surface differential is evidently a factor in de- 

 termining the new patterns: hydranth-stem axes always develop from 



Fig. 144, A-D. — Development of new polarities in blastula of the hydromedusa PhiaJidium 

 after obliteration of polarity by differential inhibition. A, solid blastula flattened on the sub- 

 strate after cessation of ciliary activity; B, hydranth-stem axis developing from upper sur- 

 face; C, hydranth-stem axis from upper surface and three stolon axes below; D, three stolon 

 axes (from Child, 1925c). 



the upper free surfaces of the masses, and stolon axes always in contact 

 with the substrate. Particular stolon axes are probably localized by slight 

 chance differences in condition in different regions, though perhaps some 

 unrecognized external differential may contribute to their localization. 

 In development from planulae under natural conditions stolon axes do 

 not appear until considerably later stages, after the hydranth-stem axes 

 has developed. Differentially inhibited planulae give very similar results, 

 though their polarities are not usually completely obliterated, and stolon 

 axes develop from one or both ends but may appear elsewhere. 



Polarity of animal eggs is usually not altered by centrifuging, though 

 position of polar-body formation may be altered by displacement of ma- 

 turation spindle or of nucleus, but ventrodorsality apparently can be 

 altered by centrifugal force in some eggs. In eggs of certain sea urchins, 



