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In the lowest of the vertebrates, Petromyzon, the two parts are distinct, the cells of the cortex-to-be are situated in the walls of the kidney blood vessels, projecting as peninsulas in the blood stream, the blood sweeping over and past them. The medulla-to-be consists of cells accompanying the vegetative nerves.
From Petromyzon upward that relationship is lost, the thyroid migrates more and more to the head region, to become the great link between sex and brain. How alive that function still is, is grossly shown by the swelling of the gland with sexual excitement, menstruation and pregnancy. Relative to the body weight it is largest in the mammalia, and smallest in the fishes.
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