EMBRYONIC RECONSTITUTIONS 513 



or less closely, but usually with defective skeleton and incomplete develop- 

 ment or absence of oral lobe and anal arms; they may attain practically 

 complete bilateral symmetry of form. 



GENERAL CONSIDERATIONS CONCERNING THE 

 SEA-URCHIN EXPERIMENTS 



Reconstitution of a whole of small size from an isolated part involves 

 decrease in scale of organization, that is, a decrease in scale of the gradient 

 system. Development is usually slower in these reconstitutions than in 

 controls, and, according to Tyler (1933), more energy is required to attain a 

 particular developmental stage than in the normal whole. That the slower 

 development and greater energy requirement of the isolated parts is not 

 a matter of the time required for "regulation" is indicated by the fact 

 that giants resulting from fusion of two fertilized eggs develop more rap- 

 idly than embryos from single eggs. The range of decrease in scale of 

 organization is rather narrowly limited in the sea-urchin embryo; conse- 

 quently, isolated parts frequently develop into forms differing in pro- 

 portions of differentiations from higher and lower gradient-levels or par- 

 tial forms in which scale of organization is larger than the isolated part. 



The paralleHsm between disproportionate and apical partial forms in 

 sea-urchin reconstitution and in postembryonic reconstitutions of hy- 

 droids and planarians and many annehds is striking, and decrease in scale 

 of organization by inhibiting conditions has essentially similar effects in 

 both. In fact, the isolations and transplantations of early sea-urchin de- 

 velopment seem to show that the primary apicobasal patterns concerned 

 in them and in the postembryonic reconstitutions of lower invertebrates 

 are not fundamentally very different. The embryonic reconstitutions of 

 the sea urchin bring to Hght little that dift'ers essentially from the data 

 of reconstitution of mature individuals. 



It has already been pointed out that two opposed substance gradients 

 may be associated with a single activity gradient and that the latter may 

 be the effective factor in development (p. 241). The assumption of two 

 opposed specifically different metabolisms coexisting in the same region, 

 each tending to "suppress" the other but each supposed to persist unless 

 completely suppressed, and appearing as required to account for experi- 

 mental results, involves difficulties. Even if two opposed concentration 

 gradients of substances are present, a single metabolic gradient may be 

 expected as a resultant. Moreover, the evidence from other reconstitu- 

 tions indicates that physiological activity, rate or intensity of metabolism, 



