Development of the Intestinal Tract during the Larval Period of Drosophila

Abstract

ABSTRACT A few hours before the end of the second larval instar the first pair of lymph glands hypertrophies, then releases cells into the haemocoele. This is quickly followed by the growth of the primordia of the more posterior pairs of lymph glands. The cellular contents of the first pair of glands then disintegrates nearly completely and the glands are regenerated from small primordia lying at their posterior end. A similar but less well-marked process takes place at the end of the first larval instar. At the time of the hypertrophy of the first pair of lymph glands in the second larval instar (and to a lesser extent also in the first instar) the main epithelium of the mid-gut hypertrophies, giving off a large number of lumps of cytoplasm, mostly without nuclei, into the lumen of the gut which becomes temporarily filled up with them. When the more posterior lymph glands develop this hypertrophy ceases, the cytoplasmic masses cytolyse and are largely resorbed by the mid-gut epithelium. Rather similar processes of hypertrophy occur in the imaginal rudiment at the anterior end of each salivary gland and in the imaginal rudiment of the hind-gut which lies just caudal to the entrance of the Malpighian tubules. Lying beneath the main epithelium of the mid-gut in the newly hatched larva are scattered small cells referred to as basement cells. It seems likely that these are derived from the lymph glands during the end of the embryonic period. Towards the end of the second larval instar the basement cells are joined by further lymph-gland cells. During the third larval instar these multiply, forming small imaginal primordia each consisting of a nest of deeply basophilic cells, the more peripheral cells throwing out long processes which extend round and enclose the others. Towards the end of the third larval instar, when the lymph gland hexagons become melanized and degenerate, the basement cells also suffer degeneration though not melanization. At the time of puparium formation the central cells of the imaginal primordia proliferate to form the wall of the imaginal mid-gut. The peripheral cells of the imaginal primordia are thrown off into the gut-lumen where they form a continuous layer around the degenerating main larval epithelium.