226 J- S. HUXLEY. 



that at metamorphosis there is present a new organic system, the 

 developing Echinus rudiment, which was absent in the subjects 

 of the dedifferentiation experiments. We may suppose that the 

 metamorphic changes in Echinoids are normally initiated some- 

 what as follows : the inherited constitution of the animal leads 

 to the production of the Echinus rudiment. The weight of this 

 leads directly to the sinking of the larva to the bottom. The sub- 

 sequent changes would then be due to two causes: (i) The con- 

 ditions at the bottom are directly unfavorable to the larval organs, 

 which therefore are unable to maintain themselves, and so start 

 dedifferentiating. (2) The developing adult organs are not in- 

 hibited by the benthic environment, and their continued growth 

 and consequent demands for nutrition accelerate the dissolution 

 of the larval system. 



The stomach is an organ which is remodelled. It is unable to 

 maintain itself in its typical larval form, and regular dedifferentia- 

 tion-changes start in it. But the activities of the adult organs 

 provide a new internal environment, and the remains of its tissues, 

 entering into equilibrium with this, form the rudiment of the adult 

 stomach. 



It should follow from these considerations that precocious meta- 

 morphosis should be induced by placing larvse with a developing 

 Echinus rudiment in dilute toxic solutions. This conclusion it is 

 intended to test by experiment. Meanwhile Professor MacBride 

 informs me in conversation that those of his cultures in which 

 conditions are not optimum, do as a matter of fact exhibit preco- 

 cious metamorphosis. In such conditions, the larvae sink to the 

 bottom while the Echinus rudiment is still in a stage much less 

 advanced than that which it possesses at metamorphosis in the 

 best cultures. In spite of this, metamorphosis takes place, but 

 leads to the production of small, weakly, under-developed Echini. 

 I am grateful to Professor MacBride for informing me of this 

 confirmation of my theoretical considerations. 



Loeb ('oo) made the suggestion that the retrogressive changes 

 of histolysis in metamorphosis were comparable to dedifferentia- 

 tion as seen in Hydroids, but I am not aware that the similarity 

 between dedifferentiational and metamorphic changes in one and 

 the same species has yet been pointed out, as here in Echinus. 



