are, and the only difference to be noted in this respect is that the process takes place within the very plasma itself, while the other elements are parted from it by the thin membrane of the capillary vessels. The globule has so distinct an existence of its own, that the chemical principles composing it are not found in its plasmatic medium. The various reactions shown by these little bodies in presence of chemical agents lead to the belief that they are found in the plasma in all stages of their development; their dimensions are not the same at the different ages of our organization. When the human germ is in process of evolution, the first lineaments of the vessels are traced in the depths of the tissues, and the heart begins to beat; the sanguine fluid is then formed, but the globules it contains are much larger than they will be after birth and in adult age. During this embryonic life the newly-formed blood does not communicate with the vessels in the maternal system—the two circulations being juxtaposed, but independent; there is not, as the belief in the seventeenth century was, a natural transfusion of the mother's blood into that of the embryo, because the solid particles or globules of each circulate and remain in each of unequal size. The study of the blood-elements in the animal series is interesting; they are found larger in fish and reptiles than in birds and mammals, whose vital activity depends on other powers. In spite of the resemblances presented by the sanguine fluid in these different groups, the blood of a fish could not vivify the body of a reptile for any length of time, nor could a bird's blood be substituted for a mammal's. The animal species whose nutritive fluid is mutually transfused must be closely related as regards natural classification: the globule which emigrates into a foreign medium can only become acclimated there in so far as its conditions of existence are not profoundly modified.
The blood-globule not only lives its individual life within the plasma, but it needs, in order to complete its function of vivifying every part of the body, to absorb oxygen from the air, and it then takes that bright vermilion color which is characteristic. The phenomenon of that new coloring is an essentially vital act, a chemical reaction taking place between two bodies, one solid, the other gaseous. Precisely the same thing happens with the commonest copper coin placed in contact with the air it absorbs a gas, and its surface is soon covered by a colored product. In the lower animals that have copper in their blood, the vine parasite, for instance, the globules take a bluish color on contact with air. The same phenomenon is remarked in the vegetable kingdom; indigo, which is white in the plant, turns blue when exposed to the air, and many coloring substances are formed in the same way. The red globule contains iron, and the chemical action taking place in it may perhaps be compared to the formation of rust. Exposed to atmospheric air, it takes a dark-red color, while continuing crimson in the arteries. Deep in the tissues, the