100 / " ; '- I." I''* "f }'n i-iiit inn. 



attrmlfd with any unfavourable result. Thus some of tin- -aine stock 

 of impregnated ova used in the second of the above experiment^ \\eic 

 kept for respectively five and twenty-four hours in on Unary tank 

 water, and were then transferred to diluted water, and kept there for 

 the remainder of their development. The larvae so obtained were 

 respectively 2 '6 and 2 '8 per cent, larger than the normal, or but 

 slightly smaller than the larvae kept for the whole period of develop- 

 ment in diluted water. In another instance, also, embryos kept for 

 respectively twenty-three hours and two days in normal water, and for 

 the rest of development in diluted water, were 3'2 per cent, and Tl per 

 cent, larger than the normal. 



These experiments therefore prove that the condition of salinity is 

 not a favourable one for the determination of the reaction to environ- 

 ment. Still they serve to emphasise the extraordinary sensitive m-^ 

 of the embryos to their environmental conditions, and also show that 

 this sensitiveness is much greater in the earlier stages of development 

 than in the later ones. Only one observation was made on the effect 

 of keeping the ova in diluted water for an hour at the time of impreg- 

 nation. In this case a diminution of 2 '2 per cent, was produced in the 

 size of the larvae. In the paper already mentioned, however, five 

 experiments of this nature are recorded,* the water being of various 

 degrees of salinity. The effects produced were respectively - 4*3, 

 + 4'1, -1'8, -2'9, and - 2'4 per cent., or on an average -1'5 per 

 cent. 



Summary. 



By keeping the impregnated ova of the Echinoid Stfongylocentntiu 

 lividus for various periods during development at an abnormal tem- 

 perature, and comparing the size of the larvae into which they developed 

 with that of larvae allowed to grow throughout under normal con- 

 ditions, it was proved that the permanent effect of temperature on flie gfoidh 

 ifi/iiiiiiislied rapidly and regularly from the time of impregnation onward*. 

 For instance, it was found that exposure of the ova to a temperature 

 of about 8 a for an hour at the time of impregnation produced an 

 average diminution of 4'1 per cent, in the size of the larvae measured 

 after eight days' growth ; during the 4th hour after impregnation the 

 diminution produced for each hour's exposure was about 1'2 per cent., 

 and during the 15th hour about - 2 per cent. In another series, 

 exposure to a temperature of 22 produced an increase in size, this 

 amounting to about I'l per cent, for each hour's exposure in the 4th 

 hour; to 0*4 per cent, in the 14th hour; 0'13 per cent, in the 46th 

 hour, and O'Ol per cent, in the 120th hour. 



Exposure to a temperature of 26 during the first few hours of 

 development produced a diminution of from 20'8 to 7 '4 per cent., 



* ' Phil. Trans.,' B, 1895, p. 588. 



