III. 5 



HEAT 



105 



rise to abnormalities accompanied by acceleration (Fere, Mitro- 

 phanow). 



The effects of extremes of heat and cold upon the ova and 

 embryos of certain Invertebrates have been studied by Driesch, 

 the brothers Hertwig, Vernon, Sala, and Greeley. 



d e f 



FIG. 56. The effect of heat upon the segmentation of the Echinoid 

 egg. a, 6, c, d, four successive stages in the segmentation of the same egg 

 of Echinus ; e,f, two successive stages in the division of the same egg of 

 Sphaerechinus. (After Driesch, 1893.) 



FIG. 57. Suppression of cell-, but not of nuclear, division by 

 heat (Echinus). (After Driesch, 1893.) 



The first-named observed that by subjecting the fertilized ova of 

 Sphaereckinus to a temperature of 30-31 (the normal is 19) de- 

 velopment was accelerated and segmentation abnormal (Fig. 56). 



After the first furrow though not after subsequent divisions 

 the blastomeres separated and sometimes remained apart, a fact 

 which provided a means of watching the independent develop- 

 ment of the first two blastomeres. After the four-celled stage 

 the direction of division became irregular, one spindle being 



