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When a 2 percent fresh solution of tannic acid is applied to frog's blood it causes the appearance of a sharply defined little knob, projecting from the free surface (Robert's macula): the coloring matter becomes at the same concentrated in the nucleus, which grows more distinct. A somewhat similar effect is produced on the human blood corpuscle.

Effect of Tannin on Red Blood Cells

When a 2 percent fresh solution of tannic acid is applied to frog's blood it causes the appearance of…