15.
E M B R Y O G E N E S I S in
vitfO
547
substance (jV) to result in neural and sensory deuterencephalic and
spino-caudal formations (Fig. 8).
Toivonen's theory is supported by the recent work of Saxen and Toivonen (1958, 1961). If one could have at one's disposal the two substances
in their pure state, one could mix them in varying proportions in order
to make a comparative study of the effects of these combinations.
Given the impossibility of obtaining, in the present state of knowledge,
these substances in their pure state, however, investigators have
circumvented the difficulty in the following manner.
They showed, first of all, that a strain of HeLa cells, in the fresh
state, induced deuterencephalic and spino-caudal formations, and that,
after heating, it induced only the archencephalon. The two inducers,
the fresh and the heated cells, can be mixed in definite proportions
FIG. 8. Diagram of two gradients of induction. At the top, the cephalo-caudal gradient with
two substances, one mesodermizing (M) and one neuralizing (N). At the bottom, the mediolateral gradient where both these same substances are active. A, Archencephalic region;
D, deuterencephalic region; S, spino-caudal region; Arch, anterior brain; B , balancers;
O c , optic vesicles; PI., placodes. (After Toivonen and Saxen, 1955.)
A
D
S
E M B R Y O G E N E S I S in
vitfO
547
substance (jV) to result in neural and sensory deuterencephalic and
spino-caudal formations (Fig. 8).
Toivonen's theory is supported by the recent work of Saxen and Toivonen (1958, 1961). If one could have at one's disposal the two substances
in their pure state, one could mix them in varying proportions in order
to make a comparative study of the effects of these combinations.
Given the impossibility of obtaining, in the present state of knowledge,
these substances in their pure state, however, investigators have
circumvented the difficulty in the following manner.
They showed, first of all, that a strain of HeLa cells, in the fresh
state, induced deuterencephalic and spino-caudal formations, and that,
after heating, it induced only the archencephalon. The two inducers,
the fresh and the heated cells, can be mixed in definite proportions
FIG. 8. Diagram of two gradients of induction. At the top, the cephalo-caudal gradient with
two substances, one mesodermizing (M) and one neuralizing (N). At the bottom, the mediolateral gradient where both these same substances are active. A, Archencephalic region;
D, deuterencephalic region; S, spino-caudal region; Arch, anterior brain; B , balancers;
O c , optic vesicles; PI., placodes. (After Toivonen and Saxen, 1955.)
A
D
S