26 D T+D D ~ D= RADIATION -INDUCED DELAY D Na R= INDUCED RECOVERY PERIOD QD A —ty, _ T+R+D' J} a oO 5 oO istic shape of this radiation sensitivity curve si gested its use in determining the timeof action of t agents. If the chemically-induced mitotic delay tal. place during the time of treatment, then it would expeeted that the radiation sensitivity curve of t drug-treated zygotes would be.shifted in time to t right by an amount equivalent to the treatment p i © T aon Oo 7 RADIATION —INDUCED CLEAVAGE DELAY IN MINUTES Fic, 20.—Schematic experimental protocol. JT represents the normal control cell division time, D the delay because of radiat exposure of the sperm, # the prolongation of normal control cell division cycle by treatment of fertilized egg with physical or che ical agent, and D’ the radiation-induced delay after such treatment. 5000R 30- CONTROL DIVISION TIME 20+ | lO 0 10 to 20 30 aL 40 50 ot 1 60 70 MINUTES AFTER FERTILIZATION Fic. 21—The sensitivity to radiation-induced cleavage delay as a function of the time during the first cell cycle the Arbacia zygotes are irradiated with 5000 R. division delay occurs in early prophase. If the zygote is irradiated after this time, no delay is evident until the second division cycle. This can be seen in Figure 21 which depicts the variation in the amount of divi- sion delay produced by a dose of radiation as a func- tion of the time during the first cell cycle that the zygotes are irradiated. For Arbacia punctulata xzy- gotes the radiation insensitive portion of the cycle begins about 15 min before division. The character- riod or its induced delay. Therefore, the fertilized eg or zygotes were treated with a numberof agents al then exposed to radiation. The changes in shape ai position of the sensitivity curves indicated that di ferent agents acted at different times. It was al> found that treatment with several agents modified t] subsequent response of the system to radiation. Th effect will be called modification and should be di tinguished from protection which implies that tl agent is present at the time of irradiation. A study ¢ this type should give some insight into the types o reactions and cellular activities that are responsib! for the variation of radiation response during the ce evele. MATERIALS AND METHODS Two species of sea urchin have been used, Arbact punctulata,* and Lytechinus pictus. The animal. are maintained in two 25-gallon refrigerated aquaris which recirculate artificial sea water (Instan’ *Supplied by the Marine Biological Laboratory, Wood. Hole, Massachusetts or Glendle Noble, Panama City, Florida + Supplied by Pacific Biomarine Supply Co. Venice, California,