THE FORMATION AND EARLY DIFFERENTIATION OF SEA URCHIN GONADS

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Abstract
1. The genital primordium gonads were examined at several stages during their development in the juvenile sea urchin. 2. At metamorphosis the primordial germ cells are a distinct group of cells in the dorsal mesentery, which also supports the stone canal and axial organs. They spread circumferentially along the aboral body wall to form a ring, the genital rachis, completely encircling the anal region. The gonads form as swellings in each interradius of this ring. 3. While the wall of the genitial primordium prior to rachis formation consists of a single layer of epithelial cells, that of the young gonad has two such layers separated by a fluid-filled space. 4. The primordial germs cells of each stage examined are similar in nuclear morphology, cytoplasmic inclusions, and cell shape and size. All stages have germ-cell-specific fibrillo-granular structures, which we name goniosomes. 5. A second cell type, the forerunner of the accessory cell of the mature gonad, is present in the genital tissue of all stages studied. Cellular inclusions characteristic of the accessory cell, including glycogen granules, lipid-like vesicles, and lysozymal-like vesicles, begin to accumulate in these cells even before gonad formation.

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