262
J E A N B R Ä C H E T
FIG. 7. Dorso-ventral R N A gradients in neurectoderm and chordomesoderm in young
neurula of Xenopus.
Mention should also be m a d e of t h e interesting results, in t h e s a m e
field, o b t a i n e d b y R o u n d s a n d Flickinger (1958), who m a d e chemical
estimations of protein a n d R N A in various fractions recovered b y
centrifugation of h o m o g e n a t e s from isolated p a r t s of gastrulae a n d
neurulae. T h e m a i n result was t h a t b o t h R N A a n d proteins are
solubilized from t h e yolk platelets in t h e invaginating chordomesoblast
during gastrulation. Since this solubilization occurs only in t h e chordomesoblast, t h e results indicate t h a t t h e release of R N A a n d protein from
t h e yolk m i g h t p l a y a direct role in neural induction, a p o i n t which will
soon be discussed.
I t h a s been generally a s s u m e d t h a t R N A a n d proteins are m e t a -
bolically inert before gastrulation. R e c e n t , still u n p u b l i s h e d work, from
our l a b o r a t o r y casts some d o u b t on this a s s u m p t i o n : for instance,
Bieliavsky h a s observed a strong incorporation of amino-acids i n t o t h e
proteins of dissociated cells from morulae, t h e nucleus being more active
t h a n t h e cytoplasm. On t h e o t h e r h a n d , in 'pulse e x p e r i m e n t s ' w i t h
3 2P - p h o s p h a t e , Decroly a n d Cape found considerable incorporation of
3 2P into R N A as early as t h e m o r u l a stage. T h e incorporation r a t e shows
a definite p e a k a t t h e t i m e of neural induction (late gastrulae, early
neurulae) a n d decreases afterwards, a t t h e t i m e of cytodifferentiation.
T h e overall base composition of t h e newly synthesized R N A ' s r e m a i n s
t h e same during t h e whole development.
J E A N B R Ä C H E T
FIG. 7. Dorso-ventral R N A gradients in neurectoderm and chordomesoderm in young
neurula of Xenopus.
Mention should also be m a d e of t h e interesting results, in t h e s a m e
field, o b t a i n e d b y R o u n d s a n d Flickinger (1958), who m a d e chemical
estimations of protein a n d R N A in various fractions recovered b y
centrifugation of h o m o g e n a t e s from isolated p a r t s of gastrulae a n d
neurulae. T h e m a i n result was t h a t b o t h R N A a n d proteins are
solubilized from t h e yolk platelets in t h e invaginating chordomesoblast
during gastrulation. Since this solubilization occurs only in t h e chordomesoblast, t h e results indicate t h a t t h e release of R N A a n d protein from
t h e yolk m i g h t p l a y a direct role in neural induction, a p o i n t which will
soon be discussed.
I t h a s been generally a s s u m e d t h a t R N A a n d proteins are m e t a -
bolically inert before gastrulation. R e c e n t , still u n p u b l i s h e d work, from
our l a b o r a t o r y casts some d o u b t on this a s s u m p t i o n : for instance,
Bieliavsky h a s observed a strong incorporation of amino-acids i n t o t h e
proteins of dissociated cells from morulae, t h e nucleus being more active
t h a n t h e cytoplasm. On t h e o t h e r h a n d , in 'pulse e x p e r i m e n t s ' w i t h
3 2P - p h o s p h a t e , Decroly a n d Cape found considerable incorporation of
3 2P into R N A as early as t h e m o r u l a stage. T h e incorporation r a t e shows
a definite p e a k a t t h e t i m e of neural induction (late gastrulae, early
neurulae) a n d decreases afterwards, a t t h e t i m e of cytodifferentiation.
T h e overall base composition of t h e newly synthesized R N A ' s r e m a i n s
t h e same during t h e whole development.