1. Experiments involving amputation and irradiation have been made on the fresh-water planarians Dugesia lugubris and Polycelis nigra to ascertain the precise roles of head and tail (Fig. 1) in the induction of a pharyngeal zone and of a pharynx.

  2. During the normal regeneration of a prepharyngeal cephalic fragment (region I, Fig. 2a), a pharynx regenerates in the original tissues of the fragment at the same time as the caudal blastema appears. During the normal regeneration of a postpharyngeal caudal fragment (region III, Fig. 2b), the pharynx appears in the new tissues of the blastema. The eyes of the regenerated head have by then already appeared.

  3. The caudal blastemata of cephalic fragments were cut away every day or every second day. A pharynx appears after a few days in the fragment (Fig. 3a). The cephalic blastemata of caudal fragments were cut away every second day. No pharynx regenerated under these conditions (Fig. 3 b).

  4. A zone close to the cut was irradiated with X-rays, which prevents the fragments regenerating for a long time. A pharynx appears in the cephalic fragment long before the regeneration blastema. In the caudal fragments, the pharynx cannot regnerate before the blastema has formed. It appears only after the blastema has differentiated into a head (Fig. 5).

  5. The same results have been obtained in another species of planarian, Polycelis nigra.

  6. The regeneration of the pharynx is therefore possible in a cephalic fragment, devoid of any blastema. The regeneration of a pharynx is impossible in a tail without cephalic, prepharyngeal or pharyngeal regions.

You do not currently have access to this content.