ASYMMETRY IX THE STARFISH. 133 



One of the principal values accruing from experimental embry- 

 ology is that it often throws light upon the mechanism of normal 

 development. It would be strange, then, were we to fail to find 

 in this study no suggestion as to a more complete understanding 

 of the significance of asymmetry in the echinoderms and of that 

 remarkable series of changes involved in metamorphosis. I 

 venture, therefore, to conclude this paper with an attempt to 

 reinterpret in physiological terms the phenomenon of echinoderm 

 metamorphosis in general. 



V. A PHYSIOLOGICAL INTERPRETATION OF ECHIXODERM 



METAMORPHOSIS. 



There are many remarkable ontogenetic transformations in 

 nature, but none so radical as the metamorphosis of the echino- 

 derm larva into the adult. In the metamorphosis of the cater- 

 pillar into the butterfly, the axes of polarity and of symmetry are 

 carried over unchanged from larva to adult ; in the metamorphosis 

 of the tunicate tadpole larva into the adult sea-squirt there is a 

 relative inhibition of the apical regions of both axes, yet the 

 polarity and symmetry of the larva are maintained in the adult; 

 but in the metamorphosis of the echinoderm both polarity and 

 symmetry of the larva are, as it were, entirely ignored, and the 

 axes of the adult are established practically de novo and without 

 much reference to previously existing axiate relations. 



In terms of the physiological gradients involved, it may be said 

 that what apparently happens is, that the original gradients of the 

 egg and of the larva become practically obliterated and a new 

 major axis arises approximately at right angles to the larval major 

 axis with its apical region at a point somewhere near the middle 

 of the left side of the larval stomach at the point where the 

 hydroccele arises. The crucial event in this shifting of axiate 

 relations is, morphologically speaking, the differentiation of the 

 hydroccele. This structure, as all know, arises as an outpouching 

 of the posterior end of the left anterior ccelom (enterocoele). 

 There comes a time in late larval life when all other growth 

 changes cease and the enlargement and differentiation of the 

 hydroccele seem to be the main changes taking place. This 

 rapidly growing region then assumes dominance over the rest of 



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