> treatr treat-
29
i.v., It enters and leaves the cell in strict conformity
to its concentration in the medium at any time.
Therefore, the effect of nitrogen on the response curve
was tested. The results are shown in Figure 29. The
reduction in radiation response after the bubbling of
is the nitrogen through the cell suspension has ceased and
h case air has been readmitted can only mean then that the
chems from
system has been modified in its response to radiation.
It can be envisioned that some reactions, sensitive to
radiation, have proceeded and perhaps have been
completed during the anaerobic period. Therefore,
"A (10
ct the bthey may no longer be as vulnerable to exposure to
his in- radiation. These reactions, however, do not seem to
ivision be those that determine the shape of the response
as obwh in
le gath two
lower
ion of
WeVEFr,
Hownicals
Ss was
lion in
f proresent
diate,
S are
curve. Only the magnitude, therefore, and not the
shape of the radiation response curve is altered by
the absence of oxygen.
CONTROL —
DIVISION
TIMES
j——25'—_| +
Figures 30 and 31 show the results of similar ex-
periments involving prolongations of the cell cycle
I time by temporary reductions of temperature. A lowering of the normal (~22° C) development tempera-
KA
‘ture, even by less than 10° C, postponed the processes
| determining the characteristic shape of the radiation
response curve but did not modify the magnitude of
the response. Since the difference in control egg divi| sion times was appreciably less than the treatment
time, however, these reactions appear to have procecded more rapidly than normal once the system was
‘returned to control temperatures. Again, therefore,
certain activity must have been possible during the
; period of reduced temperature which permitted the
0
0
|
|
i
20
30
40
30
60
§=6—70
MINUTES AFTER FERTILIZATION
10
80
4
30
Fig. 24—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 immersion in 99+% deuterated sea water for 20 min right after
fertilization.
more rapid rate of completion of the cell cycle once
RADIATION-INDUCED CLEAVAGE DELAY IN MINUTES
he cyred by
RADIATION-INDUCED CLEAVAGE
DELAY IN MINUTES
,
appear
' treatdiation
pie ke et Be ee RR Rede edb
50
]
|
CONTROL
DIVISION
TIMES
1
31 ——— |
=
Y
0
10
20
30
40
50
60
70
|
80
90
100
MINUTES AFTER FERTILIZATION
Mig, 25.—Thesensitivity to radiation-induced cleavage delay as a function of the postfertilization time Arbacia zygotes are irradinted. The dashed curve shows the effeet of treatment with 2.5 * 107? M sodium azide for 30 min right after fertilization.