TEMPERATURE ALTERATIONS OF RESPONSE 

 OF CELL DIVLSION TO URETHAN 



Ivor Cornman, Hazleton Laboratories, 

 Falls Church, Virginia 



HE PRECISE TIMING of cchinoderm egg cleavage makes these eggs con- 

 veniently useful in rate studies. But, being whole cells — in fact, whole 

 eggs — they are complex, and this complexity obscures the view when one is 

 looking for the basic mechanisms. A great many things affect the division 

 of these eggs (cf. Harvey, 4), and it is inconceivable that all influences 

 retarding cleavage act in the same way. The egg obliges with some visual 

 clues, however. Within its limited repertoire there are distinctive patterns 

 of response. The pattern induced by carbamates is distinctive, although 

 not unique. It does permit one to say with some conviction that most of 

 the simpler carbamates act in the same way, despite considerable variation 

 in molecular build (2). 



One recognizable part of the pattern is the extent to which cleavage can 

 be slowed without destroying the egg. Figure 1 shows this in the Echin- 

 arachnius egg. First cleavage was at about 110 minutes, and second cleav- 

 age at 170 minutes at 20.2 ± 0.1°C. Eggs exposed to 2.2 niM reached 50% 

 first cleavage at four hours and did not complete the first cleavage until 

 about six hours after fertilization. Yet they persisted and formed blastulae 

 the next day. Half the concentration produced a two-minute delay of first 

 cleavage ; twice as much blocked cytoplasmic division completely in some 

 of the eggs, but some went on to produce irregular ciliated cell clumps. 

 This is quite in contrast to a mitotic poison like colchicine or podophyllin. 

 One can establish a threshold dose of these substances that will [iroduce 

 a slight delay and permit continued cleavage, but any slight increase in 

 concentration blocks division completely and eventually destroys the egg. 

 One cannot get a prolonged retardation with these metaphase-blocking 

 poisons without completely disrupting ontogenesis. 



One aspect of the urethan curve in figure 1 is at once confusing and re- 

 vealing. The cleavage percentage reached 60% at 154 minutes, then 

 dropped to 22%. This is another phenomenon typical of the carbamates — 

 and again, not restricted to them. At the higher doses that still permit 

 cleavage, there is an initial spurious division. To all appearances the eggs 

 have divided, and one must count them as two-cell when following a popu- 

 lation of living eggs. Subsequently these supposedly complete furrows 

 widen into shallow grooves and eventually disappear, leaving a binucleate 



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