THE RED BLoon-CELts. These are minute bi concave disks, averaging 7 to S mieromillimeters, or about 1-3200 of an inch in their long diameter. and about one-fourth that in thickness. In adult human blood these cells are not nucleated, al though in the human embryo they contain nuclei. This round, diseoid, non-nucleated red blood-cell is characteristie of mammals with the exception of the camel family, in which the red blood-cells are non-nucleated, but oval. In birds, reptiles, amphibians, and fisheS (with the exception of Cyelostomata, which have nucleated, but round cells) the red blood-eells are oval in shape and contain nuclei. As regards size, the largest cor puscles are found in the amphibia; those of the Amphiuma being as much as 75 micromillimeters in diameter, so that they may he distinguished by the unaided eye. The following table shows the size of the red blood-cells in some of the more common mammals: Elephant .0092 millimeter man .0079 " Dog .0071 •• Rabbit .0070 Mouse .0067 •• • Horse 0059 " In normal human blood there are about 5,000,000 red blood-cells per cubic millimeter of blood. As a general rule woman's blood has somewhat fewer than mans. In different ani mals the number of red cells varies greatly, and the number of eells•per cubic millimeter is much reduced in the lower animals, which have very large corpuscles. The redness characteristic of blood is not evident in a single corpuscle, which appears of a light straw-color. When, however,
a large number of the cells are seen together, the mass appears bright-red. The red blood-corpus cles are embryologically of mesoblastie origin, be ginning their life-history as nucleated cells and as such possessing amceboid movement and re produeth'e powers. At birth all nuclei have been lost as well as the powers of movement and of reproduction. After birth the production of new red corpuscles is believed to occur mainly in the marrow of bones. The question as to the possi bility of determining by microscopical examina tion whether a specimen of blood, particularly a blood-stain, is human or animal, often presents itself in trials for murder. By reference to the table given, it will be seen that the red corpuscles in a considerable number of animals so closely approximate in size those of man that a positive answer to the question is often impossible. Then the drying of the corpuscles in a clot, which has to be softened with salt solution before examina tion can be made, introduces an uncertainty as to whether such corpuscles preserve their nor mal dimensions. On the other hand, it will be noted that a differentiation between human blood and that of lower animals whose red corpuscles are oval and nucleated—for example, birds and fishes—is comparatively easy.