Abstract
Presumptive ectoneuroderm of late blastulae or early gastrulae of Rana pipiens was partially activated by short-lasting disaggregation in Ca-free Holtfreter or Niu-Twitty solutions and subsequent reaggregation in normal solutions. The explants usually became dumbbell shaped and consisted respectively of an adhesive gland portion, a connecting mass of atypical epidermis, and
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an epidermal vesicle with neural structures. The very susceptible ectoneuroderm of early gastrulae of Ambystoma punctatum could be partially activated by simply rearing the explants in normal Holtfreter or Niu-Twitty solution. “Double” sandwiches obtained after adhesion of the inner surfaces of the flattened ectoneurodermal caps of two early gastrulae, as well as “single” sandwiches formed after the adhesion of a single flattened ectoneurodermal cap to a collodion membrane, formed very regular ring-shaped peripheral activations.
These activations segregated first into a central neural mass, an intermediate strongly thickened sensory layer, which may be called the “placodal cell material,” and an outer thin epithelial layer of the ectoderm. Subsequently the placodal material concentrated in definite placodal anlagen while the neural mass segregated into telencephalic, diencephalic, and eye analgen. This sequence of events strongly suggests that the formation of the “placodal cell material” represents an intermediate step between “neural” and “epidermal” development in the primary activation process. The subsequent concentration of this placodal material in definite placodal anlagen suggests a local secondary inductive action from certain parts of the neural formation.
The experimental material shows that a minimal size of the neural mass is required for segregation into telencephalic, diencephalic, and eye anlagen. Below this minimal size only a telencephalic anlage is formed.
The artificial activations show a very simple pattern of organization. They usually segregate into a single telencephalic anlage accompanied by a single olfactory placode (although in the Rana pipiens material complex olfactory organs are also formed) a single diencephalic anlage, and a single eye rudiment.
The quantitative composition of the segregated prosencephalic formation shows a pronounced variation. The telencephalic anlage seems to be dominant in the Rana pipiens as well as the Ambystoma punctatum material. The pattern formation in the artificial activations seems to agree with the hypothesis put forward by Boterenbrood (1962) for pattern formation in reaggregates of anterior neural plate.
There exists a highly significant positive correlation between the total volume of the telencephalic anlage and the volume of the accompanying olfactory placodes.
The very incomplete evagination of the eye rudiment, which is accompanied by a poor development of the tapetum epithelium, may be due to the absence of neural crest cells in the artificial activations.
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