[Morphological and biochemical characterization of the developmental stages of fertilized eggs inSphaerechinus granularis lam : I. Rearing, Morphology and determination of stages].

1. Rearing to the developmental stage. Eggs and embryos of the sea urchin speciesSphaerechinus granularis were kept at a concentration of 3000 per ml in 101 batches at 22° C. They were stirred under continuous aeration to counteract sedimentation for up to 72 h. The development in the population could be kept nearly synchronous all the time. 2. Morphology. Morphology has been followed up to the pluteus stage-72 h after fertilization. At the 64 cell stage no cell coronas of macromeres, micromeres and mesomeres could be discerned in this embryo. The time periods of the different stages, their corresponding volumes and their cell numbers have been determined. 3. Electronic characterization of the stages. Embryogenesis has been followed using the Coulter Counter with size distribution plotter, the volume distribution curves being recorded continuously. The sudden elevation of the fertilization membrane manifests itself as a volume increase of 50 per cent. In the course of further development the volume increases 300-400 per cent. Gastrulation events cause a drop in embryonic volume of about 150 per cent. In the course of further development two additional smaller volume peaks occur, followed by a steep rise in volume, at least up to 72 h. The results obtained from electronic methods when compared with results from microscopy prove their relevance, the data from the Coulter Counter being the more consistent ones. The electronic method makes it possible to characterize the embryos according to their developmental stage and to their pattern of population distribution in a fast and reliable fashion.