Early development in the velvet worm Euperipatoides kanangrensis Reid 1996 (Onychophora: Peripatopsidae)

We present here a description of early development in the onychophoran Euperipatoides kanangrensis with emphasis on processes that are ambiguously described in older literature. Special focus has been on the pattern of early cleavage, blastoderm and germinal disc development and gastrulation. The formation of the blastopore, stomodeum and proctodeum is described from sectioned material using light and transmission electron microscopy as well as whole-mount material stained for nuclei and gene expression. The early cleavages were found to be superficial, contrary to earlier descriptions of cleavage in yolky, ovoviviparous onychophorans. Also, contrary to earlier descriptions, the embryonic anterior-posterior axis is not predetermined in the egg. Our data support the view of a blastopore that becomes elongated and slit-like, resembling some of the earliest descriptions. From gene expression data, we concluded that the position of the proctodeum is the most posterior pit in the developing embryo. This description of early development adds to our knowledge of the staging of embryonic development in onychophorans necessary for studies on the role of developmental changes in evolution.

► Cleavages are superficial and appear identical to that described in the South-African yolk-less ovoviviparous species. ► The initial positioning of the anterior to posterior axis is not fixed in the egg. ► We propose the terms anterior and posterior part of the blastopore to replace older terms. ► The expression of the gene wingless shows the position of the proctodeum. ► This study has clarified the processes that occur in stage 1 of embryonic development in onychophorans.