Bipartite axiation follows incomplete epiboly in zebrafish embryos treated with chemical teratogens.

Medial clefts in the axis of the trunk region are malformations known from many chordates and are mostly referred to as rachischisis anterior. In teleosts, rachischisis was previously ascribed either to secondary rifting of a single uniform axial rudiment, or to the establishment of two (half) axes and body halves physically separate from the very beginning. In order to decide between these conflicting interpretations, we treated zebrafish embryos during blastodisc stages and epiboly with several chemical teratogens causing rachischisis anterior.

Embryogenesis in a Drosophila mutant expressing half the normal segment number.

Insect segments are seriated developmental subunits whose borders largely control intra-segmental patterning. The individual segment owes its specific character to position-dependent instructions given to its early embryonic progenitor cells. In Drosophila, the segment borders first become visible as a series of transverse infoldings on the surface of the germ band.

The cytoplasmic architecture of the egg cell ofSmittia spec. (Diptera, Chironomidae) : II. Periplasm and yolk-endoplasm.

1. Eggs of the midgeSmittia were investigated by light microscopy and transmission electron microscopy. This paper describes elements and architecture of periplasm and yolk endoplasm before the formation of pole cells.

The development of Drosophila embryos after partial u.v. irradiation.

U.v. irradiation of the anterior pole of nuclear multiplication stage Drosophila eggs produces embryos with defective anterior structures.

A dicephalic monster embryo ofDrosophila melanogaster.

A monster embryo with a head on each end developed from an abnormalDrosophila egg produced by a M(1)o/Binsn female. The embryo probably represents a longitudinal mirror duplication of head and all 3 thoracic segments. The significance of this type of pattern anomaly for embryonic pattern specification inDrosophila is discussed.

Pattern specification in the insect embryo.

Specification of developmental pathways by specific determining substances prelocalized in the egg cytoplasm is duscussed using the so-called germ cell determinants as an example. Some theoretical considerations speak against the assumption that in insects the various elements of the basic body plan are specified by a prelocalized mosaic of specific determinants. Experimental evidence also points towards a largely epigenetic mode of pattern specification.

The cytoplasmic architecture of the egg cell ofSmittia spec. (Diptera, Chironomidae) : I. Anterior and posterior pole regions.

1. The egg of the Chironomid midgeSmittia spec. has been studied by light and electron microscopy.

Mode and timing of body pattern formation (Regionalization) in the early embryonic development of cyclorrhaphic dipterans (Protophormia, Drosophila).

1. Eggs of the blowflyProtophormia spec. were separated into anterior and posterior fragments of varying sizes.

Pattern formation in longitudinal halves of leaf hopper eggs (Homoptera) and some remarks on the definition of "Embryonic regulation".

1. Eggs of the leaf hopperEuscelis plebejus F. were fragmented longitudinally by a novel technique during cleavage.

[Development of the malformation "double abdomen" in eggs of smittia parthenogenetica (dipt., chironomidae) partially irradiated by UV].

UV-irradiation (maximum intensity at 253,7 nm) of the anterior 1/3 of the egg during nuclear migration stage frequently is followed by the formation of "double abdomens". The type of malformation shows a germ band in which head, thorax and anterior abdominal segments are replaced by a set of 6-8 posterior abdominal segments joined in mirror-image symmetry to the corresponding parts of the original abdomen. The course of development leading to this malformation is visibly aberrant from the blastoderm stage to germ band segmentation only.