The Effect of Removing Various Endocrine Glands on the Hair Cycles of Black Rats1

The manner in which Norway rats grow their fur and replace it throughout life has been described in a previous paper (Dieke, 1947). It was shown there that in this species the hair follicles near the midventer are the first to become active, then those on the sides and finally those on the head and the rump, so that a complete new coat is acquired in a wave-like fashion, with only the follicles in certain circumscribed areas being active at any given time. Each complete wave of hair growth is called a hair cycle. In black rats the pigment which will color the hair can be seen in the skin just before the hair shafts emerge, which makes it easy to recognize and follow the active phase of each cycle. In rats of this laboratory the first hair cycle begins several days after birth and is complete at about three weeks of age;