RECONSTITUTIONAL PATTERNS IN EXPERIMENT 375 



tion of polar pattern is usually not altered in the early embryonic recon- 

 stitutions, as far as known; but there are cases in which it cannot be de- 

 termined whether it is altered or not. There is often considerable change 

 in scale of organization when half an egg or an isolated blastomere becomes 

 a complete individual. What happens to the pattern in these cases? Vari- 

 ous interpretations have been offered, in terms of hypothetical formative 

 substances, of substance gradients and their changes, etc. But if sub- 

 stances do change their positions in definite and orderly manner in these 

 cases, these changes must be determined by a pattern of some sort. What 

 is the character of that pattern? Does the reconstitution of an apical re- 

 gion on the basal half of an eight- or sixteen-cell sea-urchin embryo differ 

 fundamentally in any way from reconstitution of a hydranth in a piece 

 of Tubiilaria or Corymorpha or a head on a planarian piece? Does not 

 failure of the apical halves of the same stages to develop mesenchyme and 

 entoderm present the same problems as failure of isolated hydranths or 

 heads to reconstitute more proximal or posterior parts? With a large scale 

 of organization a short hydroid or planarian piece may develop only the 

 apical or anterior part of an individual. With decrease in scale it may 

 become an entire individual. The same is true for the apical half of the 

 sea-urchin embryo. Under natural conditions it forms only the more api- 

 cal portions of an individual. With experimental decrease in scale it may 

 become a complete individual (p. 357). 



Ventrodorsality and dorsiventrality may be reconstituted in definite 

 relation to the pattern already present, though in many cases of blasto- 

 mere reconstitution it is not known whether or not this is the case. Sup- 

 pose, for example, we isolate the blastomeres of the two-cell stage of a 

 form in which ventrodorsal pattern of some sort is present at this stage but 

 the first cleavage plane has no definite relation to it. What happens when 

 either or both blastomeres develop as entire individuals? Is the entire 

 ventrodorsal pattern present on one side and reconstituted on the other, 

 or is only a part of it present and the rest reconstituted, or is a new ventro- 

 dorsality established on a smaller scale? If ventrodorsality is primarily 

 a gradient pattern, any one of these effects is possible. If ventrodorsaHty 

 or dorsiventrality has developed beyond the simple gradient stage, recon- 

 stitution of a complete individual may be possible only with isolations at 

 or near the median plane. 



Embryonic reconstitution is narrowly limited or practically absent in 

 some forms, apparently in consequence of highly stable regional differ- 

 ences in the cytoplasm. Similar limitations appear in the adults of various 



