Certain Lavs of Variation. 85 



*' Certain Laws of Variation. T. The Reaction of Developing 

 Organisms to Environment." By H. M. VERNON, M.A., M.D., 

 Fellow of Magdalen College, Oxford. Communicated by 

 Professor E. HAY LANKESTER, F.E.S. Eeccived March 7, 

 Head March 29, 1900. 



In a former paper* it was shown that the ova of the Echinoid 

 Strong i/locentrotus limdus were extraordinarily sensitive to their environ- 

 mental conditions at the time of impregnation. For instance, by keep- 

 ing the mixed ova and spermatozoa in water at about 26 or 8 C. for an 

 hour, the plutei obtained after eight days' development were some 5 per 

 cent, smaller than those from ova kept at about 20 at the time of 

 impregnation. It was even found that the effect produced was nearly as 

 great if the time of subjection to the abnormal temperature were 

 reduced to one or three minutes, though if reduced to ten seconds it 

 was not so great. This latter result was probably due to the time 

 being insufficient for all the ova to become impregnated at the abnormal 

 temperature. 



These observations have now been repeated and confirmed, and in 

 .addition others have been made upon the reaction of the ova to 

 environment in the later stages of their development. It has thereby 

 been found that the degree of this reaction diminishes in more or less 

 regular proportion from the time of impregnation onwards. 



The method of experiment is fully described in the above-mentioned 

 paper, so it will be sufficient to state here that it consists in shaking 

 pieces of the ovaries and of the testes of several specimens of the 

 Echinoid in small beakers of water, and then bringing portions of the 

 contents to the required abnormal temperature. These portions are 

 then mixed, and after an hour the temperature is gradually brought 

 to the normal by floating the beakers in large vessels of water. The 

 now impregnated ova are then poured into covered jars holding 2| to 4 

 litres of water, and after eight days the plutei into which they develop 

 are killed and preserved, and measured under the microscope with a 

 micrometer eye-piece in groups of fifty. In addition to these plutei, 

 others are obtained in each case from ova impregnated at a normal 

 temperature, but allowed in all other respects to develop under similar 

 conditions. These constitute the normal or standard larvae, from which 

 the variations in the mean size of the other larvae are calculated. The 

 particular dimension measured was the length of the calcareous skeleton 

 of the " body " of the larva, that of the arms as a rule not being deter- 

 mined in the present research. 



The results obtained, both in the old and the present series of experi- 



* ' Phil. Trans./ B, 1895, p. 577. 

 VOL. LXVII. U 



