The life of form: emergent patterns of morphological transformation.

The inherent logical difficulties of understanding biological form in terms of historical narratives and genetic programmes are considered. It is concluded that neither of these approaches can resolve the problems presented by the evidence of parallel and convergent evolution, and for intrinsic constraints on morphogenesis such as the occurrence of phylotypic stages in the developments of members of different phyla. An alternative approach which could resolve the conflicts of historical and structural analysis is described in terms of the complex dynamics of genetic networks acting within the context of morphogenetic fields, from which emergent biological forms arise. This is described as the life of form within the biological realm.

Dwarfism in a sabellid polychaete, a study of an interstitial species.

The anatomy and behavior of a species of the sabellid polychaete subfamily Fabricinae, is described. Mature individuals are virtually microscopic and represent the extreme degree of dwarfism apparently possible for a sabellid. Eggs are matured and liberated unilaterally, one at a time, from a single thoracic segment. Dwarfism in this species is essentially a cessation of body growth at a juvenile stage, accompanied by precocious sexual maturity, as an adaptation to persistent occupation of an interstitial detrital habitat.

Induced segmental reorganization in sabellid worms.

Processes of regeneration and reorganization are analyzed in two sabellid polychaetes. Abdominal pieces of Branchiomma nigromaculata, during head regeneration, ordinarily reorganize only a fraction of the number of segments typical of the thorax. In pieces transected in the vertical plane, but obliquely with regard to the main axis of the worm, the extent of reorganization is greatly enhanced. The same number of segments transform on the right and left sides, the surviving damaged segments transforming on one side and a corresponding number of more posterior, undamaged segments transforming on the other. Reorganization is shown, in abdominal pieces of Sabella melanostigma, to involve destruction and inversed dorso-ventral reconstitution of all parapodial structures in the segments affected, the conversion of intestine to thoracic stomach, and the invasion of previously abdominal segments by a pair of large nephridia which grow posteriorly after being formed in the basal portion of the head blastema. All three events exhibit a time-graded character, staring soonest at the anterior end and progressively later posteriorly, and apparently independently of one another. Abdominal type segments are formed only from the anterior region of the caudal, prepygidal zone of growth, successively, never by transformation. The dorso-ventrality of abdominal segments is the inverse of the thoracic, with the antero-posterior polarity unchanged, in all circumstances. Only during posterior regeneration from thoracic segments are thoracic segments produced from the posterior zone of growth, but only two or three are thus formed, the zone of growth then changing to the production of abdominal segments. It is concluded that the feature of sabellid-serpulid organization is the complete inversion of the dorso-ventrality of the posterior zone of growth as the result of emancipation from the generally dominating dorsal field emanating from the anterior end.