ASYMMETRY IN THE STARFISH. 137 



expressed in the unilateral appearance of the hydrocoele and its 

 derivatives, is traceable to a very early stage of development and 

 is probably present in the unsegmented egg. Moreover, it 

 appears merely to consist of an inequality of metabolic rate 

 between the antimeric halves of egg and embryo. The difference 

 between the two sides is in no sense qualitative, as some writers 

 prefer to believe, but is purely quantitative; for, as we have seen, 

 all the structures that characterize the superior side may be 

 inhibited on that side and may be thus allowed to appear on the 

 other side, or they may appear on both sides at once in case the 

 physiological state of the two sides is experimentally equalized. 

 The basis of asymmetry in the typical echinoderm is then the 

 result of a dynamic imbalance between the earliest primordia of 

 the prospective antimeric halves of the embryo. On this account 

 one side develops more rapidly than the other, especially in the 

 region where the hydroccele arises, and the differentiation of the 

 hydrocoele and its derivatives tends to suppress the growth of 

 equivalent structures on the opposite side. This view is con- 

 sistent with the experimental work reported in this paper and 

 with a large mass of morphogenetic work of other sorts. No 

 claim of finality is made for the conclusions reached in this study, 

 but it is hoped that some of the ideas expressed may form a 

 starting point for future investigation. 



BIBLIOGRAPHY. 



Gemmill. J. F. 



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