382 Mr. H. M. Vemon. Effect of Environment [Feb. 28, 



remains, as is admitted, to be experimentally tested), the observed 

 phenomena imply a selective destruction in the one case, and not in 

 the other. 



It is not contended that the law of frequency at various ages, 

 adopted in the report, is exact. It is, however, hoped that the ap- 

 proximation is sufficiently exact to give numerical estimates of the 

 quantities measured, which are at least of the same order as the 

 quantities themselves, and for this reason it is hoped that the method 

 adopted may prove useful in other cases. 



III. " The Effect of Environment on the Development of 

 Echinoderm Larvae : an Experimental Inquiry into the 

 Causes of Variation." By II. M. VERNON, B.A. Com- 

 municated by Professor J. BuRDON SANDERSON, F.R.S. 

 Received December 10, 1894. 



(From the Zoological Station, Naples.) 

 (Abstract.) 



The conditions of environment under which an organism develops 

 are known to be of considerable influence in the production of 

 variations. It was thought to be of interest to determine by exact 

 measurement the effects which such slight changes in the environ- 

 mental conditions as might occur naturally would produce in the 

 growth of some organisms. The animal chosen was the larva or 

 plnteus of the sea-urchin Strongylocentrotus lividus. These larvae 

 develop readily from artificial fertilisations, and they can, more- 

 over, be obtained at all times of the year, irrespective of season. 

 The method adopted was to shake pieces of the ovaries and testes of 

 several sea-urchins in small jars of water, and then mix the two 

 liquids. After standing for an hour, portions of the water con- 

 taining the impregnated ova were poured into jars holding 2 to 

 3^ litres of sea water, and these were transferred to a large tank of 

 running sea water. The larvae were generally allowed to develop 

 for eight days, as the aboral and oral arms reach their maximum 

 length in this time. The larvae were killed by the addition of corro- 

 sive sublimate to the water, and were then, after washing in distilled 

 water, preserved in 70 per cent, alcohol. They were mounted in 

 glycerine and measured under the microscope, the body-length, the 

 aboral arm-length, and the oral arm-length being in each case 

 measured. The larvae were measured in sets of fifties, and the means 

 taken. The aboral and oral arm-lengths were calculated as percentages 

 on the body-length. In all 10,000 larvae were measured. 



The effects of temperature on development were first studied. It 



