i84 



THE PHYSIOLOGY OF TWINNING 



aris of ''a number of abnormal plutei which had the 

 hydrocoele developed on the right side instead of in its 

 normal position on the left side." He calls attention to 

 the fact that several other authors had previously noted 

 instances ot reversed symmetry in echinoderms. Runn- 



strom (191 2) had found this 

 condition in two individuals 

 of Strongylocentrotus lividus; 

 Miiller (1850) had long ago 

 described auriculariae with 

 hydrocoele on the right side 

 only; while Mortensen had 

 observed two plutii of 

 Ophionotus hexadu show- 

 ing similar conditions. 



The purpose of Oshima's 

 experiments was to repeat 

 McB ride's method of pro- 

 ducing larvae with double 

 hydrocoeles. Using the 

 latter's procedure he ob- 

 tained about 10 per cent 

 of larvae exhibiting situs 

 inversus (Fig. 65) and about 

 2 per cent with double 



Fig. 65. — Ventral view of a 

 pluteus larva of the sea urchin, 

 Echinus miliaris, showing com- 

 plete reversal of asymmetry in the 

 hydrocoele, which appears on 

 the right side only instead of on 

 the left, the situs typical for the 

 species. (From Oshima.) 



hydrocoeles. Curiously 

 enough, however, the control cultures, which had not 

 been treated with hypertonic sea water showed the 

 same percentage of anomalous forms. It must not be 

 forgotten, however, that the cultures were reared in 

 artificially mixed sea water and that the food was scanty. 

 These conditions were sufi&cient to account for the 



