MORPHOGENESIS OF EPITOKOUS SETAE DURING NORMAL AND INDUCED METAMORPHOSIS IN THE POLYCHAETE ANNELID NEREIS GRUBEI (KINBERG)

¹ The Hopkins Marine Station of Stanford University, Pacific Grove, California

1. The development of the heteronereid setae during the metamorphosis of Nereis grubei is described. Four stages may be distinguished by the sequential appearance within the chaetogenic tissue of the several components of these compound setae. The characteristics of each stage are summarized in Table I.

2. Initiation of setal sac development results from the decapitation of even very young gameteless animals. At least a few cells of the parapodial epidermis are competent to produce morphologically complete setae in gameteless specimens, although both the cell and its product may be smaller than normal.

3. In females with oöcytes greater than about 50 µ in diameter, a detectable group of chaetogenic cells is often present within three days of decapitation; this tissue does not normally appear until the oöcytes are 95 µ in diameter.

4. Chaetogenesis already in progress may be accelerated by decapitation of animals during stage I and early stage II, when the oöcytes are under 140 µ in diameter.

5. Decapitation of animals with oöcytes greater than 140 µ in diameter did not accelerate chaetogenesis.

6. The duration of each setal sac stage has been calculated from the known oöcyte growth rate. Metamorphosis is estimated to occupy about 67 days.

7. The relationship between the setal sac stages and the stages already in the literature is examined.

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