ASYMMETRY IN THE STARFISH. 



of the two sides were really reduced to parity, we should always 

 have, as the result of icing, bilateral larvae; but we should not, a 

 priori, expect a retarding agent to effect the two sides equally. 

 On the contrary, the expectation would be that the superior side 

 would be more seriously affected than the inferior, so that when 

 recovery took place the originally inferior side would emerge from 

 the growth retarding influence first and would then behave like 

 a physiologically superior side throughout development. Two 

 difficulties immediately present themselves. Why, it may be 

 asked, are not all larvae reversed? And why do not the right- 

 handed larvae, about ten per cent, of which seem to be present in 

 each lot, reverse and become left-handed? 



The second of these queries may best be answered first by 

 admitting that there is no ground for denying that at least some of 

 the right-handed individuals, under the conditions of the experi- 

 ment, do reverse their already reversed asymmetry back to the 

 specific left-handed asymmetry. If this be taken into account, 

 we w r ould have to add materially to the observed percentages of 

 reversed larvae, for a certain percentage of the left-handed larvae 

 that are counted as unreversed are doubtless reversed right- 

 handers. But there is still a large percentage of larvae that have 

 failed to undergo symmetry reversal, and our problem is to 

 account for their failure to reverse. The only solution of the 

 problem is one that takes into consideration the well-known fact 

 that no two eggs or embryos are alike in the degree of their 

 susceptibility to inhibiting agents or in their responses to given 

 agents. It is probable that only a limited number of larvae at 

 any one time are in just that state of susceptibility the normal 

 response to which is symmetry reversal, and that these unsus- 

 ceptible types are responsible for the surplus of left-handed larvae 

 in all experimental cultures. Other larvae show other expressions 

 of growth inhibition, while still other particularly hardy forms 

 show no effects at all. 



The problem that confronts us in this connection is in no 

 essential respect different from the one that has faced experi- 

 mental embryologists ever since the first eggs or embryos were 

 subjected to conditions other than normal. It makes no 

 difference whether one uses toxic chemicals, anaesthetics, low 



