EXPERIMENTS ON DETERMINATION IN THE EARLY DEVELOPMENT OF CEREBRATULUS LACTEUS

Abstract

The prospective significance of the animal and the vegetative cells of the 8-cell stage and of the 4 layers of the 16-cell stage /an1, an2. veg1, veg2/ was studied by means of local vital staining. The 1st furrow may form any angle to the median plane of the larva. The 2 isolated half blastomeres from 1 egg may develop into pilidia with perfect bilateral symmetry. When they are less typical their abnormalities do not show from which part of the egg the dwarf larvae come. The bilateral symmetry is not determined in the early cleavage stage. Animal and vegetative fragments differentiate as parts of a mosaic, i.e. in the same way as they would have done in the normal development. Larvae composed of an1 + veg2 did not form any of the organs normally derived from the excised middle part of the egg. Fusion of an animal with a meridional half of the 8-cell stage also showed a complete mosaic development. The results indicate that in Cerebratulus the animal-vegetative determination is accomplished earlier, the determination of the bilateral symmetry later than in the sea-urchin.