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
267
FIG. 12. Formation of nervous system in an amphibian egg treated with a mixture of
RNA and ribonuclease at the morula stage.
V a h s (1962) on n o r m a l e m b r y o s also lead h i m t o t h e conclusion t h a t t h e
n o r m a l inducing s u b s t a n c e originates from t h e yolk present in t h e
chordoblast cells. This conclusion is identical w i t h t h a t of R o u n d s a n d
Flickinger (1958), which was based on a n entirely different t y p e of
e x p e r i m e n t .
However, these experiments, which strongly suggest a n i n t e r v e n t i o n
of t h e yolk platelets in inductive processes, do n o t of course rule o u t a
possible role of o t h e r cell constituents in induction. T h e fact t h a t , as
shown b y B r ä c h e t et al. (1952) a n d , more clearly, b y Y a m a d a (1961),
isolated microsomes h a v e a strong neural inducing activity, suggests
t h a t these R N A - c o n t a i n i n g granules m i g h t also be closely associated
w i t h neural induction. F u r t h e r analysis of t h e inducing a c t i v i t y of
microsomes isolated from chick e m b r y o s ( K a w a k a m i et al., 1961;
T i e d e m a n n et al., 1962) h a s shown t h a t t h e ribosome fraction which
contains only R N A a n d associated proteins (with a basic p r o t e i n as t h e
m a i n constituent) (Crampton a n d P e t e r m a n n , 1959; Spahr, 1962) h a s a
strong neural inducing activity. On t h e other h a n d , t h e whole micro-
267
FIG. 12. Formation of nervous system in an amphibian egg treated with a mixture of
RNA and ribonuclease at the morula stage.
V a h s (1962) on n o r m a l e m b r y o s also lead h i m t o t h e conclusion t h a t t h e
n o r m a l inducing s u b s t a n c e originates from t h e yolk present in t h e
chordoblast cells. This conclusion is identical w i t h t h a t of R o u n d s a n d
Flickinger (1958), which was based on a n entirely different t y p e of
e x p e r i m e n t .
However, these experiments, which strongly suggest a n i n t e r v e n t i o n
of t h e yolk platelets in inductive processes, do n o t of course rule o u t a
possible role of o t h e r cell constituents in induction. T h e fact t h a t , as
shown b y B r ä c h e t et al. (1952) a n d , more clearly, b y Y a m a d a (1961),
isolated microsomes h a v e a strong neural inducing activity, suggests
t h a t these R N A - c o n t a i n i n g granules m i g h t also be closely associated
w i t h neural induction. F u r t h e r analysis of t h e inducing a c t i v i t y of
microsomes isolated from chick e m b r y o s ( K a w a k a m i et al., 1961;
T i e d e m a n n et al., 1962) h a s shown t h a t t h e ribosome fraction which
contains only R N A a n d associated proteins (with a basic p r o t e i n as t h e
m a i n constituent) (Crampton a n d P e t e r m a n n , 1959; Spahr, 1962) h a s a
strong neural inducing activity. On t h e other h a n d , t h e whole micro-