Certain Laws of Variation. 93 



peratuve had been reached it was removed from the warm water and 

 quickly cooled down by a stream of cold water. After keeping for 

 twenty-four hours, corrosive sublimate was added to kill off any of the 

 embryos still surviving, and they were all collected in a small glass cell 

 and examined under the microscope. From the different stages of 

 development attained by the developing ova killed at the time of heat- 

 ing, and those only killed twenty-four hours later by the sublimate, one 

 could easily determine the effect of the various degrees of high tem- 

 perature. 



For ova at the time of impregnation the fatal heat temperature is 

 probably about 28'5. Thus only 31 per cent, of some ova heated to 

 27 '7 at the time of impregnation were found to have developed to 

 normal blastulse twenty-four hours later, whereas some of the same ova 

 impregnated at a normal temperature (14'2) were all found without 

 exception to have reached the blastula stage. On the other hand, in 

 another case not a single ovum out of a number heated to 30 at the 

 time of impregnation showed any sign of normal development twenty- 

 four hours later. 



As regards subsequent stages, portions of some developing ova, four 

 hours after impregnation, were heated to respectively 29, 32, 35, 

 and 38. Next day all the embryos heated to 29 and 32 had nearly 

 or quite arrived at the pluteus stage, whilst none of those heated to 35 

 or 38 had got further than the half-formed blastula stage. The fatal 

 temperature must therefore have been between 32 and 35, or say 

 32 '5. Other portions of the same stock of developing ova were 

 heated in a similar manner twelve hours after impregnation. Next 

 day 'all those heated to 29, 32, and 35 had arrived at the full or semi- 

 pluteus stage, whilst all of those heated to 38 were either normal 

 blastulae or blastulse just beginning to invaginate. The death tempera- 

 ture in this case must therefore have been about 36 '5. Still other 

 portions of the same stock of embryos were heated to various tempera- 

 tures twenty-eight hours after impregnation. They had now arrived 

 at the free-swimming pluteus stage, and hence it was quite easy to 

 determine by naked-eye observation what effect had been produced. 

 Of the plutei heated to 37, none were affected, but all of those heated 

 to 39 sank to the bottom of the beaker in a few minutes. However, 

 about a third of them had recovered an hour after, and all of them had 

 recovered several hours after. None of these plutei were heated above 

 39, so the actual death temperature was not determined ; but other 

 results showed that the death temperature is only slightly above 

 the heat paralysis temperature, so one may conclude that it was in this 

 case about 39'5. 



On heating some of the six days plutei obtained from the same stock 

 of ova, it was found that a quarter of an hour after heating, three- 

 fourths of those heated to 39 had sunk to the bottom of the beaker, 



