1. Histogenesis of the epithelium, of the small intestine of Bu, fo vulgaris japonicus from the time of hatching to the end of meta morphosis was studied with reference to tissue iron. 2. At the time of hatching the midgut runs caudalwards at the dorsal portion of the yolk cell mass. The S-shaped small in t e stine develops from the midgut at 3-4 days after hatching. At first the intestinal lumen is occluded by the blended yolk cel l s, but soon is recanalized through absorption of the yolk platel e ts. 3. In the wall of the small intestine the yolk cells undergo active mitotic division and differentiate into a layer of columnar cells at 5-7 days after hatching. The columnar cells contain la r ge amounts of yolk platelets which appear to possess some crystalline structures. The small intestine becomes gradually elo n gated untill it aquires 3 1/2 coils at the beginning of the secon d week. Its columnar epithelial cells are provided, on the surface, with the striated border. In the intestinal epithelium goblet cells and. basal cells are discernible a mong the columnar cells. 4. During the period of metamorphosis,7 weeks after hatching, most of the epithelial cells undergo degeneration and t h e small intestine becomes markedly reduced in length and diameter. The basal cells and some of the columnar cells divide through mitosis and form cell massesregeneration nidiwhich a ppear as giant cells. These masses first form cyst-like cell groups. The neighbouring ones are connected to each other, so that the definite epithelium is formed. 5. Tissue iron occurs in the form of masked iron in the yolk platelets during development of the larval intestine. After absorption of the yolk, the reaction becomes negative. No readily demonstrable iron is detected. The validity of the available histochemical methods for masked iron is discussed.
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