ol % 60|- 4 50u _ Or : a 2 wi E 52 oz CONTROL => ' a ‘40F - DIVISION TIMES — Sia 2 <I GB <_ : . Py 30+ as 1 20+#—COLCHICINE—f 30! t \.\- _ 4 oO I Gt th on hm oS 2 a O,@ AM. oT O,m@ P.M. @ 75x 1075 M COLCHICINE| @ 1.25x10°4 M COLCHICINE rok w rT CONTROL DIVISION TIMES a 80;- | ~— 92'—+| o T un 7 RADIATION- INDUCED CLEAVAGE DELAY IN MINUTES T 0 IO 20 30 l 40 50 Teme 60 7O 10] MINUTES AFTER FERTILIZATION 3 are irOnl. eam oerBE 10K mms re-. there- | “ of 0 10 l 1 5 s | 20 30 40 50 60 70 MINUTES AFTER FERTILIZATION J Fic. 29-—The sensitivity to radiation-induced cleavage delay as a function of the postfertilization time Arbacta zygotes are irradiated. The dashed curve shows the effect of treatment with nitrogen for 30 min right after fertilization. 80 terfer- | i neil UE: EN tte Tic, 28.—The sensitivity to radiation-induced cleavage dewith : lay as a function of the postfertilization time Arbacta zygotes They, are irradiated. The open circles and squares represent experi-~ ments on the same control gametes in the morning and afterion of ° noon, respectively. The solid curve is drawn through these . lesser svmbols. The dashed curve shows the effect of treatment with down ° 75 * 10°M colchicine for 30 min right after fertilization, subse- ; while the dotted curve shows the effect of 125 x 10*M colchicine treatment for 30 min right after fertilization. Ta ir exmore acting as a basic protein with the cell surface.“ The radia- * chelating activity of EDTA is presumed responsible in some way for its action. As discussed in last year’s report,‘®) comparisons of recovery rates under the action of these various agents tends to implicate some mctabolic activity in the recovery process. One indi- Eee . cation is that more recovery generally occurs during extensions of the cell division period induced by physico-chemical agents like D2O, colchicine, and EDTA » than during comparable time periods induced by met- 35r w Ld => 2 = Z > = 4 a Lu CONTROL bd Oe IMES <q a5 [ist —l3—+! ’ Q Li Oo 2 Qo 2 ] = o bE : \ <= 0 iO 200—~CO«B—Ci4—C SL \ 5s—CiaKO—Cia7—s«#B"OD MINUTES AFTER FERTILIZATION From analysis of the variations in radiation re- Fic. 30.—The sensitivity to radiation-induced cleavage delay as a function of the postfertilization time Arbacia zygotes are irradiated. The dashed curve shows the effect of incubation at 6.5° C for 30 min right after fertilization. covered. Thus, the radiation response of zygotes that tion afforded, for example, by hypoxia. Therefore, the ‘bolie inhibitors. | §ponse curves as a function of the time during the cell eyele that fertilized eggs are exposed, a “modifi: cation” effect of certain of the agents has been un- ing exposure, it cannot be the usual type of protec- have been treated with sodium azide, beta mercapto- phenomenon has been termed modification. In a sim- ‘reated counter-parts. Since this reduced response is observed at times when the agent is not present dur- sensitize fertilized Arbacia eggs to subsequent radia- ‘thanol, nitrogen, and EDTA is less than their un- ilar experiment, puromycin treatment, on the other hand, has been shown by Rustad and Burchill*® to