EXPERIMENTS ON DETERMINATION IN THE EARLY DEVELOPMENT OF CEREBRATULUS LACTEUS

¹ From the Zoötomical Institute, University of Stockholm, and the Marine Biological Laboratory, Woods Hole, Massachusetts

1. The prospective significance of the animal and the vegetative cells of the 8-cell stage, and of the layers an₁, an₂, veg₁, and veg₂ of the 16-cell stage (Fig. 1) was studied. an₁ forms the greater part of the pretrochal ectoderm, including (probably) the most anterior part of the ciliated band (Fig. 3). an₂ gives rise to the nest of the pretrochal ectoderm, and a great part of the ciliated band. veg₁ also partakes in the formation of the ciliated band, and, moreover, differentiates into sophagus and the insides of the lappets. veg₂ corresponds to the stomach. The boundary between the layers could not, however, always be determined with complete accuracy.

2. The first furrow may form any angle to the median plane of the larva.

3. The two isolated half-blastomeres from one egg may develop into pilidia with perfect bilateral symmetry (Fig. 2 B, C). When the isolated half- and quarter-blastomeres are less typical, their abnormalities do not show from which part of the egg the dwarf larvæ come. The bilateral symmetry is not determined in the early cleavage stage.

4. Animal and vegetative fragments differentiate, as far as we can judge from comparison with our results concerning the prospective significance, in the same way as they would have done in normal development: the animal (Fig. 5), and the vegetative (Fig. 6) cells of the 8-cell stage; an₁ (Fig. 7, A-D); veg₂ (Fig. 7, E); an₂ + veg₁ + veg₂ (Fig. 8, A); an₁ + an₂ + veg₁ (Fig. 8, B, C); an₂ + veg₁ (Fig.9).

5. Larvæ composed of an₁ + veg₂ did not form any of the organs normally derived from the excised middle pant of the egg (Fig. 10). Fusion of an animal with a meridional half of the 8-cell stage also showed a complete mosaic development (Fig. 11).

6. The results are discussed and compared with those with sea urchins, annelids and mollusks. The desirability of further investigations is emphasized.