SUMMARY

The interrenal tissue of the air-breathing fish, Heteropneustes fossilis, became hypertrophied during the breeding period. The interrenal cell cytoplasm became acidophilic and vacuolated, cytoplasmic granulation disappeared and the cell nuclei increased in size. The diameter of the nuclei varied from 2·07 to 3·12 μm in the non-breeding period while in the breeding season there was almost uniform enlargement (4·54 ± 0·36 μm). Variation in nuclear size was most pronounced during maturation of the gonads, i.e. in April. However, the size of larger nuclei (average diameter 6·53 ± 0·98 μm) was not significantly different from that of thesmaller nuclei (average diameter 4·61 ± 0·61 μm) during this period (P > 0·05).

The volume density of the cell components of the interrenal tissue nearly doubled in the breeding period but the relative proportions remained more or less the same. Glycogen granules, which appeared to be uniformly distributed in the interrenal cells in the non-breeding period, became aggregated and clumped in many parts of the cell during the breeding period. There was a clear increase in the chromaffin reaction in the chromaffin cells during the breeding period but the iodate reaction, indicating noradrenaline content, decreased considerably.

In the ovarian cycle there was a close relationship between gonadosomatic index, ova diameter and water temperature. There was a more than tenfold increase in the gonadosomatic index in the breeding period (1·30 in the non-breeding and 15·27 in the breeding periods). In males, the rise in the gonadosomatic index in the breeding period corresponded to that of the ovary in degree (0·22 in the non-breeding and 3·20 in the breeding periods).

In the non-breeding period the thyroid follicles were composed of squamous epithelium (epithelial height 1·2–2·2 μm) with a follicular diameter of 48–76 μm and the colloid completely filling the lumina of the follicles; features indicative of hypofunction of the gland. In the breeding period, the follicles were composed of cuboidal epithelium (epithelial height nearly three times that in the preceding period, 4·1–6·3 μm) follicular diameter was reduced (36–48 μm) and the colloid incompletely filled the lumina, thus suggesting normal or slight hyperfunction of the gland.