I. PROBLEM OF THE ORGANIZER
41
between the explanted ectoderm and the organizer of the gastrula of
Ambystoma mexicanum resulted in suppression of induction. In the same
paper he further showed that in the organizer stained vitally with
neutral red, only formed elements of the cell are stained. After the fusion
of the stained organizer with the ectoderm, the dye passes to the
ectoderm. This passage is also suppressed by the piece of cellophane
inserted between the ectoderm and organizer. The suppression of
induction by cellophane is also noted in the case of lens induction by the
optic cup in the chick embryo (McKeehan, 1951).
Brahma (1958) put a piece of membrane having either 4 ταμ pores and
0-031 mm thickness or 1-45 μ pores and 0-16 mm thickness between the
organizer and presumptive neural ectoderm of gastrulae of Ambystoma
mexicanum or Xenopus laevis, and found that no induction occurred
across the membrane. However, according to Brächet and Hugon de
Scoeux (1949) a membrane with a larger pore size (average 3-4 μ) does
not interfere with the passage of induction. In this case long, basophilic
pseudopodia of the organizer cell enter the membrane, but do not come
in direct contact with the pseudopodia extended from the ectoderm
cells.
In a series of painstaking experiments Grobstein (1955, 1956, 1957)
analysed some inductive interactions in vitro between tissues of the
mouse embryo using membranes of known porosities. His main effort
was directed to ascertain the maximum distance which the inductive
influence can traverse, when reacting and inducing layers are separated
by membranes of various porosities. According to his more recent data,
the tubule-inducing influence of the dorsal spinal cord on the metanephrogenic mesemchyme can be transmitted through a thickness of 60 μ
of filters having a porosity range of 0*4 μ~0-8 μ. In the filter, fine
processes of the cytoplasm may penetrate as deep as 8 μ or more, but
with the filter of 60 μ thickness a direct cytoplasmic connection between
inducing and reacting layers is completely excluded. In other words, the
induction is possible through the filter without direct cellular contact.
Insertion of a filter with pores of ca. 0-1 μ between the two layers allows
transmission of induction only when the filter thickness is less than 20 μ.
Also in this system cellophane constitutes an effective barrier to the
inductive influence. Working on cartilage differentiation of chick
somites under the inductive influence of the ventral part of the spinal
cord, Lash, Holtzer, and Holtzer (1957) found that the factor can
traverse a filter of 20 μ thickness and of 0-8 μ pore size. The time
required for the passage of the factor through the filter was estimated to
be ca. 8 hours.
These experiments with filters indicate that no direct contact of
inducing and reacting cells is required for the passage of induction, and
41
between the explanted ectoderm and the organizer of the gastrula of
Ambystoma mexicanum resulted in suppression of induction. In the same
paper he further showed that in the organizer stained vitally with
neutral red, only formed elements of the cell are stained. After the fusion
of the stained organizer with the ectoderm, the dye passes to the
ectoderm. This passage is also suppressed by the piece of cellophane
inserted between the ectoderm and organizer. The suppression of
induction by cellophane is also noted in the case of lens induction by the
optic cup in the chick embryo (McKeehan, 1951).
Brahma (1958) put a piece of membrane having either 4 ταμ pores and
0-031 mm thickness or 1-45 μ pores and 0-16 mm thickness between the
organizer and presumptive neural ectoderm of gastrulae of Ambystoma
mexicanum or Xenopus laevis, and found that no induction occurred
across the membrane. However, according to Brächet and Hugon de
Scoeux (1949) a membrane with a larger pore size (average 3-4 μ) does
not interfere with the passage of induction. In this case long, basophilic
pseudopodia of the organizer cell enter the membrane, but do not come
in direct contact with the pseudopodia extended from the ectoderm
cells.
In a series of painstaking experiments Grobstein (1955, 1956, 1957)
analysed some inductive interactions in vitro between tissues of the
mouse embryo using membranes of known porosities. His main effort
was directed to ascertain the maximum distance which the inductive
influence can traverse, when reacting and inducing layers are separated
by membranes of various porosities. According to his more recent data,
the tubule-inducing influence of the dorsal spinal cord on the metanephrogenic mesemchyme can be transmitted through a thickness of 60 μ
of filters having a porosity range of 0*4 μ~0-8 μ. In the filter, fine
processes of the cytoplasm may penetrate as deep as 8 μ or more, but
with the filter of 60 μ thickness a direct cytoplasmic connection between
inducing and reacting layers is completely excluded. In other words, the
induction is possible through the filter without direct cellular contact.
Insertion of a filter with pores of ca. 0-1 μ between the two layers allows
transmission of induction only when the filter thickness is less than 20 μ.
Also in this system cellophane constitutes an effective barrier to the
inductive influence. Working on cartilage differentiation of chick
somites under the inductive influence of the ventral part of the spinal
cord, Lash, Holtzer, and Holtzer (1957) found that the factor can
traverse a filter of 20 μ thickness and of 0-8 μ pore size. The time
required for the passage of the factor through the filter was estimated to
be ca. 8 hours.
These experiments with filters indicate that no direct contact of
inducing and reacting cells is required for the passage of induction, and