EFFECT OF AGING ON NEURAL COMPETENCE OF PRESUMPTIVE ECTODERM, AND EFFECT OF AGED ECTODERM ON DIFFERENTIATION OF THE ORGANIZER IN CYNOPS PYRRHOGASTER II. ROLE OF EXTREME POSTERIOR OF THE ARCHENTERIC ROOF IN THE SLIT-BLASTOPORE STAGE IN NORMAL DEVELOPMENT

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Abstract
The effect of aging on the neural competence of the presumptive ectoderm in gastrulae of C. pyrrhogaster and the effect of aged ectoderm on differentiation of the extreme posterior of the archenteric roof in the slit-blastopore stage were examined by a sandwich method in which this organizer was wrapped in the presumptive ectoderm taken from the 0-42 h aged exogastrulae. Vital staining showed that this organizer becomes mainly tail notochord. It should be called tail or trunk-tail organizer. In 0-18 h explants, typical trunk-tail structures were formed. With further aging of the presumptive ectoderm, a decrease of spinal cord and muscle with a concomitant increase of mesenchyme and mesothelium was observed. In 36 (corresponding to the slit-blastopore-initial neural stage) and 42 h explants, neural competence disappeared markedly. The notochord appeared in all explants, indicating this organizer is more firmly determined than the uninvaginated dorsal lip in small yolk-plug stage. This organizer does not play an important role in the induction of the neural plate, but induces the tail in normal development.