ABSTRACT
The post-naupliar germ bands of many higher crustaceans show a regular grid-like pattern of cells. This pattern is generated, in part but not in toto, by the proliferation of teloblasts. The exact lineage of all the cells of the post-naupliar germ band has been investigated in most of the orders belonging to the monophyletic unit Peracarida (Cumacea, Tanaidacea, Isopoda, Mysidacea, Amphipoda). The cell divisions and differentiation could be followed up to the formation of appendage buds, of ganglia and of intersegmental furrows.
The most-striking result is that the genealogical borders between cells of different clones do not correspond to the transverse intersegmental furrows. Instead, the genealogical borders divide a segment, and even the appendage buds, into anterior and posterior compartments.
There are different pathways for the formation of the cells of the post-naupliar germ band, though the subsequent differentiation may be nearly identical. It has been deduced from these findings that the fate of the cells is not determined by their origin. This supposition could be substantiated by a comparative analysis of the different orders. In Amphipoda, for instance, ectoteloblasts are not differentiated; the post-naupliar germ band is formed by an assemblage of blastoderm cells. Nevertheless, the cleavage pattern of these cells is for the most part identical to that of the other orders that possess ectoteloblasts.
