Memoirs: The Early Development of the summer egg of a Cladoceran (Simocephalus vetulus)

Abstract

1. Bach egg is laid as a yolky mass of a foam and later forms a primary egg-membrane. 2. Cleavage is completely superficial and apparently indeterminate. 3. The first differentiation of the blastoderm is the appearance of a group of vacuolated yolky cells on the ventral side of the embryo which are called the ventral mass. 4. This subsequently differentiates into a few large cells with very large nuclei which form the genital rudiment, surrounded laterally and posteriorly by ectomesodermal cells, and anteriorly to this a mesondodermal mass of cells from which arises the mesendoderm. 5. The genital rudiment surrounded laterally and posteriorly by inwardly growing ectomesodermal cells invaginates and becomes internal by the lips of the imagination growing together and fusing. 6. The mesendoderm grows backwards as a solid mass of cells, which later spreads out flat and becomes indistinguishable from the laterally-lying mesoderm, and from this layer the endoderm separates as a solid rod in the median plane.