22
CHR. P. RAVEN
conjunction of the two processes of ooplasmic segregation and cell
division, it is hardly conceivable that here, we have to deal with two
separate groups of factors. It seems rather probable that they are
connected into a unified pattern. It is this pattern of morphogenetic
cortical factors that will be indicated by the term 'cortical field'.
This term does not imply that the pattern of morphogenetic factors
is superimposed, so to speak, as a special entity upon the cell membrane
irrespective of its other properties as an organelleregulatingpermeability,
rigidity, adhesiveness, etc. It is rather probable that all these properties
of the membrane are closely linked. The cortical field then is to be
considered as the expression of local differences in the molecular
structure and physiological properties of the membrane.
VII. The Nature of the Cortical Morphogenetic Field
When eggs of Limnaea are treated at early stages with weak concentrations of lithium chloride, various malformations, especially of the
head region, are obtained (Raven, 1942). The most characteristic group
of malformations are those belonging to the cyclocephalic series. The
eyes and tentacles of the two sides, which are wide apart in normal
embryos, approach each other on the dorsal side of the head. They may
fuse in the mid-line, giving rise to cyclopic embryos. With further
increase of the effect, anophthalmic or even acephalic embryos may be
produced.
In normal development, the eyes and tentacles, together with the
cerebral ganglia, develop from two lateral areas of small ectoderm cells,
the cephalic plates. These are separated by a median band of seven flat
ciliated cells, the apical plate. Laterally and ventrally the cephalic
plates are bounded by the ciliated cells of the prototroch, dorsally, by
the large, flat, but non-ciliated cells of the head vesicle (Fig. 8A).
In the cyclocephalic embryos it appeared that, besides the displacement of eyes and tentacles, there occurred, as a rule, a shortening of the
cerebral commissure and a fusion of the left and right cerebral ganglia
(Raven, 1949). The two cephalic plates are connected by small ectoderm
cells across the mid-line. The posterior part of the apical plate, which
normally separates the two cephalic plates, seemed to be lacking in all
cases. Since this part develops from the cells immediately surrounding
the original animal pole, it was concluded that the effect of the treatment
consisted in a suppression of the differentiation of the most-animal
structures of the embryos. This was explained by the hypothesis that
the cortical morphogenetic field has the character of a gradient-field
(Raven, 1943) with the high point at the animal pole (Raven, 1949,
1952). The pattern of differentiation was assumed to be dependent on
CHR. P. RAVEN
conjunction of the two processes of ooplasmic segregation and cell
division, it is hardly conceivable that here, we have to deal with two
separate groups of factors. It seems rather probable that they are
connected into a unified pattern. It is this pattern of morphogenetic
cortical factors that will be indicated by the term 'cortical field'.
This term does not imply that the pattern of morphogenetic factors
is superimposed, so to speak, as a special entity upon the cell membrane
irrespective of its other properties as an organelleregulatingpermeability,
rigidity, adhesiveness, etc. It is rather probable that all these properties
of the membrane are closely linked. The cortical field then is to be
considered as the expression of local differences in the molecular
structure and physiological properties of the membrane.
VII. The Nature of the Cortical Morphogenetic Field
When eggs of Limnaea are treated at early stages with weak concentrations of lithium chloride, various malformations, especially of the
head region, are obtained (Raven, 1942). The most characteristic group
of malformations are those belonging to the cyclocephalic series. The
eyes and tentacles of the two sides, which are wide apart in normal
embryos, approach each other on the dorsal side of the head. They may
fuse in the mid-line, giving rise to cyclopic embryos. With further
increase of the effect, anophthalmic or even acephalic embryos may be
produced.
In normal development, the eyes and tentacles, together with the
cerebral ganglia, develop from two lateral areas of small ectoderm cells,
the cephalic plates. These are separated by a median band of seven flat
ciliated cells, the apical plate. Laterally and ventrally the cephalic
plates are bounded by the ciliated cells of the prototroch, dorsally, by
the large, flat, but non-ciliated cells of the head vesicle (Fig. 8A).
In the cyclocephalic embryos it appeared that, besides the displacement of eyes and tentacles, there occurred, as a rule, a shortening of the
cerebral commissure and a fusion of the left and right cerebral ganglia
(Raven, 1949). The two cephalic plates are connected by small ectoderm
cells across the mid-line. The posterior part of the apical plate, which
normally separates the two cephalic plates, seemed to be lacking in all
cases. Since this part develops from the cells immediately surrounding
the original animal pole, it was concluded that the effect of the treatment
consisted in a suppression of the differentiation of the most-animal
structures of the embryos. This was explained by the hypothesis that
the cortical morphogenetic field has the character of a gradient-field
(Raven, 1943) with the high point at the animal pole (Raven, 1949,
1952). The pattern of differentiation was assumed to be dependent on