CELL LINEAGE STUDIES IN THE CRAYFISH CHERAX-DESTRUCTOR (CRUSTACEA, DECAPODA) - GERM BAND FORMATION, SEGMENTATION, AND EARLY NEUROGENESIS

The cell division pattern of the germ band of Cherax destructor is described from gastrulation to segmentation, limb bud formation, and early neurogenesis. The naupliar segments are formed almost simultaneously from scattered ectoderm cells arranged in a V-shaped germ disc, anterior to the blastopore. No specific cell division pattern is recognisable. The post-naupliar segments are formed successively from front to rear. Most post-naupliar material is budded by a ring of about 39 to 46 ectoteloblasts, which are differentiated successively and in situ in front of the telson ectoderm. The ectoteloblasts give rise to 15 descendant cell rows by unequal divisions in an anterior direction, following a mediolateral mitotic wave. Scattered blastoderm cells of non-ectoteloblastic origin in front of the ectoteloblast descendants and behind the mandibular region are also arranged in rows. Despite their different origins, teloblastic and non-teloblastic rows cleave twice by mediolateral mitotic waves to form 4 regular descendant rows each. Thereafter, the resulting grid-like pattern is dissolved by stereotyped differential cleavages. Neuroblasts are formed during these differential cleavages and segmentation becomes visible. Each ectoderm row represents a parasegmental unit. Therefore, the segmental boundary lies within the area covered by the descendants of 1 row. Segmental structures (limbs, ganglia) are composed of derivatives of 2 ectoderm rows. The results are compared with the early development of other crustaceans and insects in relation to mechanisms of germ band formation, segmentation, neurogenesis, and evolution.