The infrastructure of a transplantable, pituitary tumor secreting growth hormone and prolactin has been studied from 1 to 5 weeks of growth. The tumor is a chromophobic cell carcinoma with multiple necrotic areas and numerous atypical mitoses. Invasion into surrounding muscle and skin is seen. Only one type of parenchymal cell could be identified in the tumor despite the production of two different hormones. The cell appeared metabolically active having a well‐developed Golgl apparatus, rough endoplasmic reticulum and numerous polyribosomes. Very few granules were occasionally present in the cytoplasm. Numerous atypical mitoses along with a slight polymorphism of nucleus were observable. Early degenerative changes appeared in the nucleus or cytoplasm of some cells near the periphery of some necrotic areas of tumor. With the nucleus, heterochromatln exhibited periodic margination, enchromat‐in occupying the center of the nucleus. In the cytoplasm of other cells, a sparslty of cytoplasmic organelles was obser
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