N U C L E I C A C I D S A N D S U L P H Y D R Y L G R O U P S
263
2. Experimental Effects on RNA
Gradients
T h e effects of m a n y chemicals (analogs of p u r i n e s a n d pyrimidines,
dinitrophenol, steroid h o r m o n e s , Li+ ions) on e m b r y o n i c d e v e l o p m e n t
h a v e been s t u d i e d w i t h a m p h i b i a n eggs. I n all cases, R N A synthesis a n d
morphogenesis h a v e always been found t o be closely linked. Steroid
h o r m o n e s a n d Li+ ions are of special interest because t h e y decrease or
modify t h e R N A g r a d i e n t s a n d , p r e s u m a b l y as a result, lead t o such
abnormalities as a s y m m e t r y , microcephaly, lack of chorda, etc.
Centrifuging e x p e r i m e n t s h a v e clearly shown t h a t e x p e r i m e n t a l
modification of t h e R N A g r a d i e n t p a t t e r n leads t o serious morphogenetic
troubles. F o r instance, centrifugation of t h e fertilized egg produces
strong microcephaly, while centrifugation a t t h e blastula stage is
followed b y t h e formation of double e m b r y o s . Cytochemical observations show t h a t t h e R N A distribution is changed, in t h e t w o cases, in
such a m a n n e r t h a t t h e embryological results can be easily explained
(see B r ä c h e t , 1960, for m o r e details). As a n e x a m p l e , Fig. 8 shows t h e
formation of a secondary axis in a region of t h e e m b r y o where a strong
R N A a c c u m u l a t i o n resulted from centrifugation a t t h e blastula stage.
W h e n d e v e l o p m e n t is blocked b y a ' h e a t shock' (1 h a t 37°, a t t h e
FIG. 8. Differentiation of a RNA-rich secondary embryo from a centrifuged blastula.
263
2. Experimental Effects on RNA
Gradients
T h e effects of m a n y chemicals (analogs of p u r i n e s a n d pyrimidines,
dinitrophenol, steroid h o r m o n e s , Li+ ions) on e m b r y o n i c d e v e l o p m e n t
h a v e been s t u d i e d w i t h a m p h i b i a n eggs. I n all cases, R N A synthesis a n d
morphogenesis h a v e always been found t o be closely linked. Steroid
h o r m o n e s a n d Li+ ions are of special interest because t h e y decrease or
modify t h e R N A g r a d i e n t s a n d , p r e s u m a b l y as a result, lead t o such
abnormalities as a s y m m e t r y , microcephaly, lack of chorda, etc.
Centrifuging e x p e r i m e n t s h a v e clearly shown t h a t e x p e r i m e n t a l
modification of t h e R N A g r a d i e n t p a t t e r n leads t o serious morphogenetic
troubles. F o r instance, centrifugation of t h e fertilized egg produces
strong microcephaly, while centrifugation a t t h e blastula stage is
followed b y t h e formation of double e m b r y o s . Cytochemical observations show t h a t t h e R N A distribution is changed, in t h e t w o cases, in
such a m a n n e r t h a t t h e embryological results can be easily explained
(see B r ä c h e t , 1960, for m o r e details). As a n e x a m p l e , Fig. 8 shows t h e
formation of a secondary axis in a region of t h e e m b r y o where a strong
R N A a c c u m u l a t i o n resulted from centrifugation a t t h e blastula stage.
W h e n d e v e l o p m e n t is blocked b y a ' h e a t shock' (1 h a t 37°, a t t h e
FIG. 8. Differentiation of a RNA-rich secondary embryo from a centrifuged blastula.