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Abstract
Embryos of the amphipod crustacean Orchestia cavimana are examined during cleavage,
gastrulation, and segmentation by using in vivo labelling. Single blastomeres of the
8- and 16-cell stages were labelled with DiI to trace cell lineages. Early cleavage
follows a distinct pattern and the a/p and d/v body axes are already determined at
the 4- and 8-cell stages, respectively. In these stages, the germinal rudiment and
the naupliar mesoderm can be traced back to a single blastomere each. In addition,
the ectoderm and the postnaupliar mesoderm are separated into right and left components.
At the16-cell stage, naupliar ectoderm is divided from the postnaupliar ectoderm,
and extraembryonic lineages are separated from postnaupliar mesoderm and endoderm.
From our investigation, it is evident that the cleavage pattern and cell lineage of
Orchestia cavimana are not of the spiral type. Furthermore, the results of the labelling
show many differences to cleavage patterns and cell lineages in other crustaceans,
in particular, other Malacostraca. The cleavage and cell lineage patterns of the amphipod
Orchestia are certainly derived within Malacostraca, whose ancestral cleavage mode
was most likely of the superficial type. On the other hand, Orchestia exhibits a stereotyped
cell division pattern during formation and differentiation of the germ band that is
typical for malacostracans. Hence, a derived (apomorphic) early cleavage pattern is
the ontogenetic basis for an evolutionarily older cell division pattern of advanced
developmental stages. O. cavimana offers the possibility to trace the lineages and
the fates of cells from early developmental stages up to the formation of segmental
structures, including neurogenesis at a level of resolution that is not matched by
any other arthropod system.