246 PATTERNS AND PROBLEMS OF DEVELOPMENT 



vegetal region, that is, prospective entoderm and perhaps lower levels of 

 ectoderm, undergo activation to a level above that of the apical or animal 

 half at the time of gastrulation, as various lines of evidence considered in 

 this and earlier chapters indicate, its greater injury by absence of sulphate 

 may be due to increased susceptibility, as its dissociation by lithium ap- 

 parently is; or after its activation it may produce certain metabolites in 

 larger quantity than the animal half. The modifications figured and de- 

 scribed in connection with these experiments apparently do not differ 

 essentially from those produced by other agents. The hypotheses ad- 

 vanced may be entirely correct, but the evidence on which they are based 

 does not appear adequate to exclude other possibilities. 



Summing up, it appears that much of the evidence regarded as indicat- 

 ing presence of regional specificities in the sea-urchin egg and early embryo 

 is open to other interpretations and that the data of differential suscepti- 

 bility, both of differential death and differential developmental modifica- 

 tion, and the data of differential dye reduction give no evidence of re- 

 gional specificities. In fact, they indicate that such specificities are either 

 absent in early stages or not sufficient to give rise to distinct, specifically 

 different regional effects on development with different agents. Doubtless 

 the metabolisms of ectoderm, entoderm, and mesenchyme do sooner or 

 later become specifically different. Perhaps they begin to do so from the 

 earliest stages of development or earlier; but, if so, the differences do not 

 become sufficient to fix definitively their characteristics, that is, they do 

 not become definitively "determined" until a later stage. Probably no 

 one would maintain that all the local specific differences of later stages 

 are present in the egg as actual, localized differences at the beginning of 

 development: this, of course, would mean complete predetermination. 

 But if new differences can originate and be localized during the course of 

 development, the possibility cannot be excluded that specific differences 

 of ectoderm, entoderm, and mesenchyme may not be present primarily 

 or may be so slight at the beginning of development that they have little 

 effect but increase gradually during development. In short, it appears 

 possible, and much of the evidence supports the view, that specific re- 

 gional differences arise secondarily from a primarily quantitative pattern. 

 In early echinoid and asteroid development graded differences in rate of 

 metabolism or of certain metabolic reactions appear to be much more 

 important in determining the course of development and its modifications 

 under experimental conditions than any regional specificities that may be 

 present. 



