TWINNING AND REPRODUCTION OF TWINS IN PELMATOHYDRA OLIGACTIS

¹ Department of Biological Sciences, Northwestern University, Evanston, Illinois

1. In the specimens observed in pedigreed cultures, genuine twins arose only in buds.

2. Fission, regarded as a separate regulatory process in which twinning complexes are resolved into single individuals, occurs after the twinning buds have become detached from the parent.

3. In pedigreed cultures specimens arising as single buds produced 1278 buds of which 24 were twin buds. Specimens arising as twinning buds produced 523 buds of which 19 were twin buds.

4. Twinning may occur in a bud at any stage of development. An early twinning bud is deeply divided at the time of detachment from the parent and a late twinning bud is divided only at the apex.

5. Multiple twinning occurs occasionally in which one or both members of a twin bud undergo secondary twinning before detachment of the complex from the parent.

6. Bud production by a specimen arising as a twin bud is equal to that of a single individual as long as the budding zone is undivided. Bud production is doubled as the budding zone is divided by fission.

7. Completion of fission of a twin bud requires usually from 8 to 27 days but may take as long as 51 days in a depressed specimen. Fission proceeds rapidly at the apical end, passes through the budding zone in three or four days and is retarded most at the basal end of the body stalk and the foot.