170 



Mr. H. Ohshima. Reversal of 



Mortensen.* In both these cases the abnormal larvae were found in nature, 

 not reared under artificial conditions. 



The purpose of our experiments was to repeat Prof. MacBride's method of 

 artificial production of the double hydroccele (6). Having been unable to 

 get an uninterrupted supply of enough food, which consisted of Nitzschia, 

 and from other causes unknown to us at present, we could not arrive at any 

 satisfactory conclusion so far as the effect of the increased salinity is 

 concerned. From both lots of cultures, treated with the " hypertonic " sea 

 water as well as untreated, more than 10 per cent, of the larvae exhibited 

 the situs inversus, and much less number, 2 per cent, at most, developed 

 the double hydroccele. 



There are at least three possible ways in which the situs inversus of the 

 Echinoplutei may occur : — 



1. It may be a case analogous to the reversed Gasteropods, as made known 

 by Crampton and Conklin. Some unnatural conditions, unavoidable in the 

 course of the artificial fertilisation of the sea-urchin eggs, may have changed 

 the polarity of the latter, with the result that the structure of the larva 

 assumes siich a totally reversed situs (Conklin, (1), p. 585). It may be 

 stated here in this connection that the phenomenon can hardly be regarded 

 as due to heredity. Our cultures were obtained from three different parents. 

 It seems highly improbable that we should find at least three congenitally 

 reversed individuals (more than 4 per cent.) out of seventy sea-urchins 

 which had been sent from Plymouth, and those three, those selected at 

 random for their good eggs. The remainder were not tested. If the 

 peculiarity were congenital, the occurrence of such reversed larvte in nature 

 would not be very rare. 



2. Eesults of Eunnstrom's experiments carried out with Strongylocentrotus 

 and Solaster {{10), pp. 533-44, text-figs, la, 10; (14), pp. 471-73, text- 

 figs. 16«, h) show that the larva developed from the right half of the egg or 

 embryo of these Echinoderms had an indication of being a mirror-image of 

 the normal form. If we compare this fact with Spemann's experiments on 

 the twins and double monsters of Triton, in which the situs inversus 

 occurred in most of the right side member ((15), pp. 390-93), we are led to 

 suppose that the abnormal larvae before us may have developed from those 

 eggs or embryos which had accidentally undergone dissociation. Our 

 abnormal larvae were, however, neither smaller in size nor slower in develop- 

 ment than the normal ones, so that the above supposition seems hardly 



* I have to acknowledge my hearty thanks to Dr. Th. Mortensen, who informed me 

 of this discovery and kindly permitted me to mention it here. 



