118 HIROSHI OHSHIMA 



shells. In some genera and species it is a normal character, 

 while in others it is regarded as abnormal. As is well known, 

 the sign of the reversal goes as far back as the se^grnenting egg, 

 which sliows its spiral cleavage in the direction contrary to 

 that found in the eggs which will give rise to normal dextral 

 snails. Conklin (3, p. 585) tried to interpret the phenomenon 

 by assuming the reversal of the polarity in the egg, which 

 change might have taken place in its very early stage. This 

 hypothesis, though still lacking any satisfactory experimental 

 evid(uice, is very simple and admirable ; and besides this we 

 have as yet no other explanation. 



There is no reason to deny that a state similar to that 

 occurring among sinistral Gasteropods may occur also among 

 Echinoderms. But can we not find in our cases of Echinus 

 larvae any other interpretation w'hich is more plausible and 

 more probable than this ? 



Th(^ Echinoderm egg has been known to be ' equipotent ', 

 or, in other words, the distribution of the organ-forming 

 substances becomes established nmch later than in the eggs 

 of most other groups. We owe to Hunnstrom our know- 

 ledge of this question. In his series of experiments with 

 Strongylocentrotus lividus (24, pp. 533-44, Text- 

 figs, 7 a, 10) he showed that in this species embryos developing 

 from half-eggs assumed normal characters later than did 

 similar embryos of Echinus microtuberculatus and 

 Sphaerechinus granular is. The larva developed 

 'probably' from the right half of the egg of Strongylo- 

 centrotus has its skeleton more strongly developed on the 

 right side than on the left, and, moreover, the coelomic sac 

 appeared only on the right side. Another of his experiments 

 (28, pp. 471-3, Text-figs. 16 a, h) shows that wdien an early 

 gastrula of Solaster sp. had been constricted along its 

 median line, in the double monster so produced, no hydrocoele 

 formed ; but a dorsal pore appeared on its left side instead of 

 on the right, forming a mirror-image of the dorsal pore of the 

 left half. He thus confirms what D r i e s c h observed in some 

 few double monsters of Echinus microtuberculatus 



