142 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



20°, and egg cleavage also becomes abnormal above 20°. From this we 

 generalized that the temperature at whicii urcthan acts least on cell divi- 

 sion lies close to the spawning temperature of the sea urchin. The data from 

 Tripneustes and Lytechnius fit this supposition, but we do not have enough 

 runs over a wide enough range to test it critically. 



Coming back to the point of flexure in the dose-temperature curves, 

 one can say only that it occurs at a temi^erature Arbacia prefers and the 

 one at which it breeds. It is not the maximal rate of cleavage. If one plots 

 rate of cleavage against temperature, the maximum falls somewhere 

 around 26°C (fig. 4, right-hand ordinates). This cannot be called the op- 

 timal temperature if you include normal development in the criteria, for 

 at 24°-28° cleavage is irregular in some eggs from the very beginning, and 

 development is abnormal. Perhaps 21°-22° is optimal for Arbacia develop- 

 ment, depending on what meaning you jnit on the word. 



Because a wide variety of molecular configurations produces the typical 

 carbamate response, and because urethan can be readily removed from 

 the cells, it appears that the carbamates do not combine strongly with the 

 protoplasmic constituents involved, but are loosely held in the cell by ad- 

 sorption or solution (3) . If the binding is of this latter type, then it follows 

 that as the temperature is increased, the narcotic is less easily retained in 

 the cell. Hence the direct relationship between concentration and tempera- 

 ture in the lower temperature range. Beyond the optimal temi:)erature, 

 however, heat itself produces untoward effects. Both aspects of the process 

 can be related to a single mechanism if we assume the carbamate inacti- 

 vates an enzyme, and that above a critical temperature, varying with the 

 species, heat contributes to this enzyme inactivation (5). Cytological 

 similarity of effects throughout the temperature range would support this 

 single-process interpretation. The egg and cell division are too complex to 

 permit delineation of the ultimate mechanism. It is hoped that these data 

 will provide some transition, however, between the trenchant quantita- 

 tions of enzyme systems and the bewildering biology of the intact or- 

 ganism. 



SUMMARY 



Carbamates, over a four-fold dose range, retarded the division of sea- 

 urchin eggs without disrupting the ontogenetic process. Echinarachnius 

 eggs at 20°C were slightly retarded by 11 niM urethan, and time to first 

 cleavage was doubled at 22 ibm. Carbamates of a great variety of alkyl 

 and aryl substitutions also retarded cleavage in proportion to dose and, 

 in general, the higher congeners were more effective than ethyl carbamate. 

 In the two species most extensively studied, the effectiveness of carbamates 

 varied with tcmiierature. The first cleavage of Arbacia was retarded 6% 



