260
J E A N B R Ä C H E T
FIG. 4. R N A gradient in fertilized Xenopus egg.
5, 6 a n d 7, correspond essentially t o gradients in t h e distribution of t h e
ribosomes. T h e l a t t e r are still free in t h e cytoplasm in these early stages
of d e v e l o p m e n t b u t , during differentiation, a well-defined endoplasmic
reticulum is gradually formed (Karasaki, 1959).
T h e reality of t h e animal-vegetal, dorso-ventral a n d anterio-posterior
gradients in t h e R N A distribution of blastulae, gastrulae a n d n e u r u l a e
h a s been confirmed b y q u a n t i t a t i v e estimations (Brächet, 1942;
Steinert, 1951 ; T a k a t a , 1953). More recently, Pfautsch (1960), also using
chemical m e t h o d s , found t h a t t h e R N A c o n t e n t is higher in t h e u n d e r -
laid ectoderm t h a n in t h e non-underlaid ectoderm. T h e R N A c o n t e n t
varies in t h e following order, in t h e various regions of t h e e m b r y o :
underlaid e c t o d e r m > n o t y e t underlaid e c t o d e r m > d o r s a l lip of blastopore > a r c h e n t e r o n roof. There is no difference a m o n g these various
regions in so far as D N A is concerned. I n e x p i a n t s of underlaid ectoderm,
t h e R N A c o n t e n t increases for 5 d a y s a n d t h e n decreases, p r o b a b l y as a
consequence of cellular differentiation. These chemical m e a s u r e m e n t s of
Pfautsch (1960) are in exact a g r e e m e n t with our earlier cytochemical
findings on t h e R N A gradients (Brächet, 1942).
individual cells, except w h e n t h e l a t t e r belong t o a n organ which
actively synthesizes protein (Brächet, 1942).
I t should be a d d e d t h a t these gradients, which are depicted in Figs. 4,
J E A N B R Ä C H E T
FIG. 4. R N A gradient in fertilized Xenopus egg.
5, 6 a n d 7, correspond essentially t o gradients in t h e distribution of t h e
ribosomes. T h e l a t t e r are still free in t h e cytoplasm in these early stages
of d e v e l o p m e n t b u t , during differentiation, a well-defined endoplasmic
reticulum is gradually formed (Karasaki, 1959).
T h e reality of t h e animal-vegetal, dorso-ventral a n d anterio-posterior
gradients in t h e R N A distribution of blastulae, gastrulae a n d n e u r u l a e
h a s been confirmed b y q u a n t i t a t i v e estimations (Brächet, 1942;
Steinert, 1951 ; T a k a t a , 1953). More recently, Pfautsch (1960), also using
chemical m e t h o d s , found t h a t t h e R N A c o n t e n t is higher in t h e u n d e r -
laid ectoderm t h a n in t h e non-underlaid ectoderm. T h e R N A c o n t e n t
varies in t h e following order, in t h e various regions of t h e e m b r y o :
underlaid e c t o d e r m > n o t y e t underlaid e c t o d e r m > d o r s a l lip of blastopore > a r c h e n t e r o n roof. There is no difference a m o n g these various
regions in so far as D N A is concerned. I n e x p i a n t s of underlaid ectoderm,
t h e R N A c o n t e n t increases for 5 d a y s a n d t h e n decreases, p r o b a b l y as a
consequence of cellular differentiation. These chemical m e a s u r e m e n t s of
Pfautsch (1960) are in exact a g r e e m e n t with our earlier cytochemical
findings on t h e R N A gradients (Brächet, 1942).
individual cells, except w h e n t h e l a t t e r belong t o a n organ which
actively synthesizes protein (Brächet, 1942).
I t should be a d d e d t h a t these gradients, which are depicted in Figs. 4,