III. 5 HEAT 107 



to be from 17-5 to 21-5 C. Exposure to high or low tempera- 

 tures after fertilization, either for longer or shorter intervals or 

 continuously, produced a decrease in body-length of the Plutei. 

 The arm-length, however, increased with increasing temperature. 



Vernon has also made the most interesting observation that 

 the variability alters with the temperature. Eight-day larvae 

 were measured, and the mean variability (Galton's Q) of the 

 body-length was found to be at 16 to 18, 22-2, at 18 to 

 20, 26-3, at 20 to 22, 24-8, and at 22 to 24, 24-0. Thus the 

 variability is greatest at the temperature most favourable for 

 development, and conversely. 



It is also possible for the cell processes that occur during 

 fertilization itself to be seriously affected by heat and cold, as 

 the researches of O. and R. Hertwig have shown. 



Moderate exposure (twenty minutes) of the eggs of Strongylo- 

 centrotns to a temperature of 31 C. so weakens the cytoplasm 

 that many spermatozoa are enabled to enter. Each sperm 

 forms its own aster, and these combine with one another to 

 form various irregular mitotic figures (triasters, tetrasters, and 

 so on). The segmentation of such eggs is very irregular. 

 With longer exposures the cones of entrance become feebly 

 developed and the asters are not formed, while the numerous 

 sperm-nuclei remain unaltered. Greater heat over 40 C. 

 prevents the entrance of the spermatozoa altogether. 



This pathological polyspermy may also be produced by cold ; 

 in this case also excessive exposure prevents the formation of the 

 vitelline membrane, the cones of entrance, and the sperm-asters, 

 while the spermatozoa remain in the peripheral layer of the egg. 

 The effect of a low temperature on eggs which have already 

 been normally fertilized is seen in the reduction of the astral 

 rays and spindle fibres, though not of the spheres, and in the 

 thickening and irregular aggregation of the chromosomes. At 

 a normal temperature the achromatic figure reappears. 



Very similar phenomena have been described by Sala in 

 Ascaris. This author kept the eggs (the females, that is, con- 

 taining eggs in all stages of development) at low temperatures 

 from 3 to 8 C. for from half an hour to five hours and 

 longer. The effect of a short exposure to a very low tern- 



