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A. RICHARDS. 



influence which will stimulate growth would also accelerate the 

 division rate, but the evidence is not clear that such is always the 

 case. For instance, Yung in 1881 exposed the eggs of Lunnata 

 stagualis to light of various wave lengths and found that the 

 hatching periods varied from 17 days in the case of violet to 36 

 days in that of red ; it would seem that the light must have ac- 

 celerated the division rate, but since the data were not taken with 

 this point in view, further evidence is necessary. The list of 

 agencies in which the evidence is definite includes heat, x-rays, 

 radium, thyroid secretion, supra-renal extract, alcohol, dibasic 

 potassium phosphate, potassium sulphate, potassium bromide, 

 oxygen, sodium hydroxide, and pilocarpine hydrochlorate. 



That the rate of development of an organism depends upon tem- 

 perature is a matter of common information. The development 

 is retarded at low temperatures and increased as the temperature 

 rises toward a maximum (above which, however, the rate almost 

 instantly falls off until death results). The first careful inquiry 

 into these matters was made by Oscar Hertwig ('96 and '98) upon 

 the eggs of Rana fusca. Since Hertwig many investigators have 

 studied the effects of temperature. The correlation between the 

 increase in the rate of division and the rise in temperature has 

 suggested that van't HofFs law may apply to the mitotic processes. 

 Mitosis, however, may not be looked upon as a simple reaction 

 which would respond directly to temperature rises, so the results, 

 as might be expected, have not been entirely uniform upon this 

 point. Laughlin investigated the division rates at 10, 20, and 

 30 in his studies on the onion root tip and determined the values 

 of OK, (the velocity increase at a given temperature compared with 

 the velocity of the same stage at 10 C. lower). He says: "From 

 the Oio values derived from these comparisons it is found that 

 each mitotic stage presents characteristic velocity reactions to tem- 

 perature increments. These reaction values approximate van't 

 HofFs expectations, thus indicating that most probably the reper- 

 toire of activities constituting each stage is composed of the actions 

 and interactions of those much more elementary physical and 

 chemical forces which measured in more isolated relations have 

 been shown to react in this same velocity fashion." 



The acceleration of the cleavage rates of eggs, due to their 



