In the early days of the embryo the blood vessels and corpuscles appear to be formed at the same time from the middle layer of the blastoderm (mesoblast). They first consist of round, nucleated, colorless cells, which subsequently become colored, gradually lose their nucleus, and assume the characteristic shape of the red corpuscles, the rest of the original mass of protoplasm remaining as a rudimentary blood vessel.

In the later stages of embryonic life the red corpuscles are said to be formed in the liver, possibly out of protoplasmic elements which are made in the spleen and thence carried to the liver by the portal circulation.

In the connective tissue of rapidly growing animals - tadpole (Kolliker), rabbit (Ranvier), rat (Schafer) - certain cells can be seen connected in the form of a capillary network, and within the protoplasm of these cells red coloring matter is developed, and the particles of color can soon be recognized as characteristic blood corpuscles, arranged in rows within the newly-formed networks. Thus isolated, small networks of capillaries, consist-21 ing of a few meshes filled with blood corpuscles, are formed inde pendently of the general circulation.

These corpuscles and their haemoglobin are manufactured by isolated protoplasmic elements in the connective tissue, and subsequently added to the general mass of blood by the growth of the network bringing it into continuity with the neighboring vessels.

In the adult the formation of red blood corpuscles is much less active, but never ceases to take place in health, for the corpuscles must be renewed as they become worn out and incapable of performing their function. This reproduction can go on with considerable rapidity, as we see after severe hemorrhage, when the normal richness in haemoglobin and corpuscles is soon regained. Their formation is, however, probably confined to a few special organs - spleen, liver, red medulla of bones - where transitional forms are found in such numbers as to point to the probability of the red corpuscles being the offspring of the colorless cells, whose protoplasm either manufactures anew or collects the necessary haemoglobin, and then loses its nucleus and ordinary cellular characters.

We can only guess at the fate of the discs, but there are many things which point to the spleen as the organ in which they are destroyed. In the spleen an enormous number of protoplasmic elements are produced, and the blood comes into relationship with the nascent cells in a way unknown in any other part of the body. Further, various unusual elements, some like altered red corpuscles, others like white cells enveloping haemoglobin, are found in this organ.

The blood corpuscles on coming to the spleen are possibly submitted to a kind of preliminary test of general fitness, some elements of the spleen pulp having the faculty of examining their condition and deciding upon their fate. Many, no doubt, pass the trial without any change, being found in good working order. Others that are found totally unfit are broken up, and their effete haemoglobin carried to the liver to be eliminated as bile pigment. Some possibly undergo a form of repair; a white cell taking charge of a weakly disc renews its stroma, adds to its haemoglobin, and carries it through the final proof in the liver, where it is chemically refreshed before going to the lungs for the load of oxygen which it has to carry to the systemic capillaries.