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. 2. Irrespective of the plane of fragmentation, both fragments may produce germ bands which are either of perfect bilateral symmetry or contain at least some bilaterally symmetrical body segments. With one possible exception, no longitudinal half-patterns were formed. 3. Ventral and lateral egg halves produce either complete germ bands or anterior partial germ bands. The failure to produce posterior body segments is correlated to the absence of special posterior pole materials marked by the symbiont mass. 4. Dorsal egg halves may produce either complete germ bands or posterior partial germ bands. The loss of anterior segments may be due to lack of keimanlagen cells. 5. The results are discussed and compared to those from transverse fragmentations carried out earlier. 6. Aided by a formalized presentation of the results a dilemma of definition is pointed out which results from the common usage of defining embryonic regulation as the alternative of mosaic development and at the same time by reference to the normal result of development.