Studies On the Organizer Problem in Pelmatohydra Oligactis

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Abstract
1. Determination of the hypostome occurs very early in the development of a bud (our stage I) in Pelmatohydra oligactis. 2. The mode of induction by a developing hypostome (type II) is different from that by an adult hypostome (type I). In the former case, an implant always induces a long tube-like outgrowth before any tentacle rudiment becomes visible. In the latter case, simultaneous induction of tentacles and body takes place. Frequently, tentacle rudiments appear before visible differentiation of the body. 3. Type II induction is characteristic of developing hypostomes from some phase of stage 1 up to a stage at which the bud possesses short, but non-motile tentacles (our stage 4). 4. As soon as the detachment of a young polyp from the parent occurs, there is a sudden change in the mode of induction, from type II to type I. 5. Possible explanations of the course of differentiation in the hypostome and of the transition between the two types of induction are suggested.