Some aspects of segment formation and post-placental development in Peripatus acacioi marcus and marcus (Onychophora)

The surface morphology of the anterior-to-posterior sequence of segment formation in embryos of a viviparous neotropical onychophoran and aspects of post-placental development seen using scanning electron microscopy are described. When all the segments have formed and the walking legs have completed their elongation, the body surface becomes covered with an embryonic cuticle that does not exhibit the hydrofuge properties seen in the adult cuticle. As soon as the walking legs have reached their full length, barbed projections are formed at their distal extremities. These projections are extensions of single cells and are covered by the embryonic cuticle. Transmission electron microscopy reveals that the cells at the distal ends of the legs and their projections have many pinocytotic vesicles at their surfaces. The cytoplasm of these cells and their projections is rich in mitochondria, rough endoplasmic reticulum, glycogen, and granules of storage material. There are minor differences in the surface morphology of the projections found at the ends of the walking legs in embryos of Peripatus acacioi and those of Peripatus biolleyi. The projections and the embryonic cuticle persist thoughout postplacental development. The role of the projections in the uptake of material by the embryo from the uterus is discussed and the possible phylogenetic significance of these projections is suggested.