122
HERMAN
DENIS
1.0
Unfertilized
(a)
Early cleavage
(b)
Late cleavage
8 (c)
A 9\ -
0.5
M
Gastrula
(d)
Neurula
(e)
• Tail bud
1.0
i A
0.5
M JW180
60
40
20
3000
1000
10
20 0
Tube no.
10
20
FIG. 3. Sucrose density gradient of the RNA purified from the ribosomes of
developing embryos. The RNA analyzed in each gradient was extracted from 200
embryos labeled by injecting
3 2
P into the female. Open circles—optical density;
filled circles—radioactivity.
would, therefore, begin before the eggs are layed. It would actually start
at the time of the hormonal injection that causes ovulation (Fig. 3). The
production of heterogeneous RNA remains weak but detectable during
the whole cleavage. It increases considerably at the beginning of gastrulation, at which time the synthesis of rRNA is also initiated.
A better proof of the existence in the embryos of an RNA fraction
that sediments heterogeneously in sucrose gradients and that is different
from rRNA was provided by the study of the anucleolate mutant of
Xenopus laevis discovered by Elsdale et al. (1958). This mutant does not
10
20 0
Tube no.
FIG. 4. Sucrose density gradient of the RNA extracted from anucleolate embryos
labeled with
1 4
C0 2 for 90 min at stage 30 (heart beats) and kept in nonradioactive
medium for various lengths of time. Open circles—optical density; filled circles—
radioactivity.
O-D-260
mf
L
£
10
20
0
•—• Radioactivity (cts/min)
<~> O.D.26
Q