. Contributions to embryology. Embryology. THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 81 show no evidence of fatty substances on histological examination may neverthe- less actually contain a large amount as revealed by chemical analysis, and vice versa. The mitochondria are phosphatids, not neutral fat. It may also be urged that the cell contains a large variety of phosphatids of different solubilities and that our mitochondrial methods bring to hght onh' those of a certain kind. This is undoubtedly true, but it is not the whole story, because our technique reveals all the phosphatids whic
. Contributions to embryology. Embryology. THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 81 show no evidence of fatty substances on histological examination may neverthe- less actually contain a large amount as revealed by chemical analysis, and vice versa. The mitochondria are phosphatids, not neutral fat. It may also be urged that the cell contains a large variety of phosphatids of different solubilities and that our mitochondrial methods bring to hght onh' those of a certain kind. This is undoubtedly true, but it is not the whole story, because our technique reveals all the phosphatids which occur in definite form and may be seen in the hving cell, so that we are studjang a constant, not a variable thing. A word of caution in connection ^\'ith the interpretation which we may justly place upon variations in the amount of mitochondria: In many pathological conditions there is a deposition of lipoids within the cell, the so-called lecithin metamorphosis. These must be carefully distinguished from true mitochondria. In other cases deposits of neutral fat occur which may be detected by the fact that they reduce osmic acid. Moreover, these substances almost invariably occur as spherules, very rarely as filaments, as is usually but not always the case mth the mitochondria. The total absence of mitochondria in normal actively functioning cells which are not senile (p. 79) is of considerable interest, though it is in all probability a phenomenon of very rare occurrence. ^Nlost of the examples recorded have, on further examination, proved to be erroneous; for instance, the parietal cells of the stomach, myeloblasts, the cells of malignant tumors, and others. I have never succeeded in finding mitochondria in tissue mast-cells, but I am incUned to think that this may simply be due to the acidophihc mitochondria being obscured by the densely packed basophihc granulations. They are present, however, in blood mast-cells. The cells in the kidney of snakes, described by Regaud (1
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