IVOR CORNMAN 



141 



and 14% at 12.6°. This calculation is based on the eggs' first try at division 

 which was unsuccessful, producing the dro]) in cleavage percentage shown 

 in the cui-\-e for 12.6°. These figures point to minimal sensitivity between 

 17° and 21°, or j^erhaps the curve flattens there. A series of finer tempera- 

 ture gradations are needed to pinpoint any flexure. 



Converting the previous data to relate dose to a roughly equal retarda- 

 tion of the eggs produces figure 4. To induce a uniform retardation of divi- 

 sion in sea-urchin eggs at temperatures from 12° to 21°-22°, one requires 

 progressively more urethan — at least a twofold increase. Beyond 22°, the 

 effectiveness of urethan again increases. Diethyl urethan fits this rough 



8- 



FiG. 4. Uniform retardation of 

 Arbacia cleavage. Circles repre- 

 sent approximately equal retar- 

 dation of cleavage at different 

 temperatures and different doses 

 (abscissa at left). To produce 

 this effect, the maximum 

 amount of ethyl carbamate {up- 

 per curve) or ethyl-N,N-dieth.vl 

 carbamate (lower curve) is re- 

 quired at about 21°. Because 

 the points are approximations, 

 the slopes are not accurate. 

 Cleavage rate (triangles, ab- 

 scissa at right) reaches a maxi- 

 mum at 25°-26°C. 



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scheme. Although it is about six times as potent as urethan, it shows the 

 same minimal effectiveness at 21°-22°. 



These curves do not apply to the sand dollar. Compare, for instance, 

 the marked retardation of Echinarachnius eggs at 22 niM and 20.2°C (fig. 

 1) as against slight retardation of Arbacia eggs by 55 mM at 20.6° (fig. 2). 

 The difference lies not in the greater susceptibility of Echinarachnius, but 

 in its different temperature response. It appears to be minimally affected 

 by urethan somewhere below 17°C. Resistance to diethyl urethan also is 

 shifted toward the lower temperature, so it appears that the curves for the 

 two carbamates are roughly parallel, the diethyl congener being more ef- 

 fective throughout, as in Arbacia. The results for j^henyl urethan (ethyl- 

 N-phenyl-carbamate) do not fit this pattern. Possibly its low solubility 

 introduces a complicating influence. There is, then, a species difference. It 

 appears not to be random. Echinarachnius comes from colder water north 

 of Cape Cod. It does not easily survive aquarium temperatures much above 



