The organizer and the bilateral body plan

On the following pages models are worked out that allow the generation of a near-Cartesian coordinate system in higher organisms

An ancestral body pattern evolved into the head of higher organisms
The foot of the hydra is controlled by the same genes as the heart and forebrain of higher organisms; the hydra 'head' is posterior

The marginal zone or germ ring – the spot-like AP organizer became a large ring in vertebrates
The tremendously enlarged blastopore allows a pronounced off-axis position of the blastopore-located dorsal organizer

The generation of the vegetal pole and the ecto-meso-endoderm separation as a pattern-forming process
Depending of the parameter, endo- and mesoderm cells may arise in coherent regions or in a salt-and-pepper pattern

Generation of a dorsal organizing region and a rational for the Nieuwkoop center
A pronounced asymmetry reduces the danger that supernumerary organizers are formed. This is especially important for large eggs as given in the frog

The generation of the Spemann organizer and its A-P polarity
Autocatlysis may be based on an inhibition of an inhibition; from early stages onwards an anterior and a posterior part can be distinguished

The midline problem and the vertebrate solution
The generation of a single stripe-like organizing region as required to pattern the mediolateral axis in an intricate problem; in vertebtrates a moving organizer leaves the midline in its wake

A cartesian coordinate system for the chick
A partial epiboly deforms the marginal zone to a hairpin: the primitive streak

The insect solution of the midline problem
A dorsal organizer suppresses midline formation; the midline appears ventrally and simultaneously over the whole AP axis; a possible reason for the DV-VD conversion

The flatworm solution of the midline problem
The dorsoventral pattern maybe hierarchically the primary event; the circumventing DV border obtains organizing properties

The left-right pattern in vertebrates and why this was easier to realize in vertebrates
The ‘left’ signal first contributes to midline formation, later it becomes quenched and has to escape.

Regulatory features of organizing regions
Diverse and even very unspecific manipulations can initiate the formation of supernumerary organizing regions

The role of maternally supplied asymmetries
Asymmetries lower the danger of supernumerary organizers and accelerate pattern formation

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