Prostomial regeneration in the polychaetePlatynereis dumerilii (Audouin et Milne-Edwards) (Annelida).

1. In Platynereis dumerilii, the capacity for anterior regeneration is rather limited: only prostomial regenerates can be developed, provided that a part of the original prostomium is leftin situ. At best, the regenerate represents a complete prostomium, but reduced in size. In most cases however, asymmetrical or deficient development of palpae, antennae and eyes is observed. Sometimes, multiple eye-anlagen are produced. The proportion of deficiencies increases with the quantity of prostomial tissue removed. 2. As demonstrated by the simultaneous emergence of typical posterior regenerates, the formation of prostomial regenerates is essentially linked with the presence of brain hormone producing cells in the persisting portion of the supraoesophageal ganglion. 3. Individuals, the dissected prostomia of which are in addition deprived of the brain rudiments, can form prostomial regenerates if brain hormone is supplied by prostomia of immature donors, implanted into the coelom. It is therefore concluded that the competence to form a prostomium is inherent in the dermal-muscular tunic of the prostomium. 4. Regenerated prostomia originating from parts of the prostomial dermal-muscular tunic under the hormonal influence of implanted prostomia exert typical functions. They enable the animals to take up food and, if the donor's prostomium is removed, to perform endocrine control of the animal's development: typical posterior regenerates are formed upon caudal amputation, also maturation and heteronereid transformations occur.