GENEOLOGICAL. 387 



(e) Influence of Temperature. In his account of 

 the development of one of the earthworms (Allolobo- 

 phora trapezoides) , whose eggs very frequently form 

 twins, Vejdovsky suggested that the " twinning " was 

 perhaps influenced by warmth, for it was most fre- 

 quent in warm weather. This suggestion prompted 

 Driesch to try the effect of increased warmth on the 

 developing eggs of the sea-urchin (Sphosrechinus) . 

 The usual result was very striking, though it was 

 not quite constant, nor verifiable in related forms, 

 e.g., Echinus. What often happened in the case of 

 S pliasrechinus was the formation of distinct twin- 

 embryos and even twin-larva? (Plutei) from each 



egg- 



In a later series of experiments, Driesch showed 

 that when the blastula-embryos (hollow balls of 

 cells) of Splioerechinus granularis are kept in sea- 

 water on a stove heated to about 30C., the great 

 majority show in about 18 hours a remarkable state 

 of affairs (exogastrula-state) in which the area of 

 cells which is normally invaginated to form the 

 primitive gut, bulges outwards instead of inwards. 

 The final result, which may survive for a week, is a 

 gut-less larva an " Anenteria." 



Many other experiments, both as to heat and cold, 

 have been made, and they are probably of great im- 

 portance since vicissitudes of temperature are of 

 frequent occurrence in natural conditions. It may 

 be conjectured that the temperature influences the 

 metabolism of the cells, e.g., the rate of formation 

 of nuclein-compounds, and thus affects the manner 

 of growth. 



(/) Influence of Chemical Re-agents. In 1887, 

 O. and R. Hertwig published a pioneer-research on 

 the influence of chemical and other stimuli on fertil- 



