ASYMMETRY IN THE STARFISH. I2Q 



IV. DISCUSSION OF EXPERIMENTAL RESULTS. 



Patiria miniata is a species of starfish that either exhibits in 

 nature a fairly high degree of reversed asymmetry or else is so 

 susceptible to influences tending to affect the symmetry system 

 that even control cultures cannot be reared without the induction 

 of some degree of symmetry reversal. Three experiments were 

 tried with the idea of determining to what extent symmetry 

 reversal could be reduced by taking every conceivable precaution 

 against conditions that might be subnormal. Using only the 

 freshest eggs and a minimum sperm concentration, allowing only 

 a few eggs to a dish, keeping the dishes in running sea-water, 

 feeding the larvae on a pure culture of the diatom, Nitschia, and 

 introducing some fresh sea-water daily, there was found to be on 

 the average about 8 per cent, of larvae showing reversed asym- 

 metry, which is not apprecially smaller than that seen in the 

 controls in the various lots previously dealt with. One may 

 conclude then that Patiria is a species in which left-handed 

 asymmetry is not fully fixed, but that the symmetry system is in 

 a somewhat plastic state, a state well adapted for experimental 

 purposes. 



Left-handed asymmetry is at the present time normal for 

 echinoderms. Considerable work has been done which tends to 

 prove that this asymmetry is present even in the unsegmented 

 egg, and also that the first cleavage divides the egg into the 

 prospective right- and left-hand sides. When twins are produced 

 by the isolation of the blastomeres at the two-cell stage it has been 

 found that one twin develops faster and becomes a more advanced 

 larva at any given time than does the other. It seems then that 

 one half of the egg, one of the two blastomeres, and one half of the 

 larva is physiologically superior to the other in the sense that it 

 has a higher general metabolic rate as expressed in more rapid 

 growth. As a rule the superior side is the left side. 



Studies of the development of double monsters, which are 

 incompletely divided twins, have shown that, in these bilateral 

 systems, whenever one component for any reason gains a physio- 

 logical ascendancy over the other, there is a strong tendency for 

 the superior component to remain normal and for the inferior 

 individual to become subnormal and to be reduced to a condition 



