CROSSINGOVER IN DROSOPHILA 163 



testing factors in the second chromosome, bottles kept between 

 19°C. and 27°C. will show a negligible deviation in percentage of 

 crossingover due to temperature, and may be safely compared, 

 other things being equal. This is reassuring to most workers 

 with Drosophila for ordinary room temperature seldom varies 

 beyond these limits. 



The curve is of very great theoretical interest in another 

 connection. A glance at its form is sufficient to convince anyone 

 that we are not dealing here with an ordinary chemical reaction 

 following van't Hoff's law, such as is commonly found in con- 

 nection with many physiological processes. The amount of 

 crossingover is not approximately tripled by a rise in tempera- 

 ture of 10°C, as we should expect if the process were comparable 

 to those tested by Snyder, 1908, and other workers in this field. 

 The observed increase in crossingover reaches a maximum at 

 about 13°C. and a second higher maximum at 31°C. Between 

 these two a minimum is reached which continues with little 

 alteration for 8 or 9°. Such a curve would seem to indicate some 

 sort of a change in the physical state, such as might be met with 

 in a study of certain colloidal phenomena. 



There is an extremely interesting parallelism between the 

 curve given above for the alteration of the percentage of cross- 

 ingover and that which physiologists have worked out for the 

 amount of contraction of various types of muscle when given 

 the same stimulus at different temperatures. This fact has been 

 called to my attention by Mr. R. H. Bowen of this laboratory. 

 In his Textbook of Physiology, Howell gives such a curve for the 

 height of contraction of frog's muscle in which the amount of 

 contraction increases gradually from zero degrees Centigrade to 

 9°C. where it reaches a first maximum. The amount of con- 

 traction then decreases, reaching a low point at between 16°C. 

 and 20°C. It then rises rapidly, reaching a higher maximum than 

 the first at about 28°C., after which it continues to decrease until 

 rigor mortis sets in at 38°C. Such a striking correspondence, 

 even in the exact temperatures, would seem to be more than 

 mere coincidence. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 24, NO. 2 



