(1) The so-called ‘luteal’ cells of the ovary are derived from the medullary cords of the embryo, as previously shown by Firket and Nonidez. The process, which appears to be a peculiar form of fatty infiltration, begins about the eleventh day of incubation.

(2) Multiplication of the cells occurs in post-embryonic life and, as suggested by Nonidez, appears to take place not by mitosis but by a neoformation at the expense of certain epithelial elements in the stroma and ovarian thecae, which are derived from the distal extremities of the medullary cords.

(3) The fully formed ‘luteal’ cell is seen to have a rather small spherical nucleus and vacuolated cytoplasm, which impregnates very deeply with the Mann-Kopsch and Da Fano techniques, and which contains blocks and granules of mitochondrial origin. With Benda's stain fat can sometimes be demonstrated in the vacuoles. Golgi bodies are situated round the archoplasm.

(4) The formation of ‘luteal’ cells in cases of sex reversal is found to be a repetition of the embryonic process. Sex cords are proliferated into the ovarian stroma and either give rise to seminiferous tubules or remain undifferentiated and form typical ‘luteal’ cells. In the latter case usually many of the neighbouring cords are degenerating.

(5) The mode of origin of the ‘luteal’ tissue in the embryos, normal adult and cases of sex reversal, is difficult to reconcile with Morgan's theory of the endocrine activity of these cells in relation to plumage.

(6) In view of Geoffrey Smith's work on Inachus and Sacculina, it is suggested that the exhibition of hen-feathering may possibly be due to a high lipoid content of the blood. On this assumption, the association of female plumage with the presence of ‘luteal’ cells in the gonad would be explicable, since inactive tissue such as the medullary cord cells would be more prone to fatty degeneration in such hen-feathered individuals.

(7) It is thought that Pearl and Boring are mistaken in homologising the ‘luteal’ islets with the cells of the mammalian corpus luteum, since the numerous vacuolated cells of discharged and large atretic follicles are found to be mainly the product of fatty degeneration of ordinary connective tissue elements, while the yellow pigment seems to be derived from aborting blood vessels. This pigment is regarded as hsematoidin and is not found in the true ‘luteal’ cells.

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