Cleavage in embryos of the sea urchin Arbacia punctulata consists of eight very rapid divisions that require continual protein synthesis to sustain them. This synthesis is programmed by stored maternal mRNAs, which code for three or four particularly abundant proteins whose synthesis is barely if at all detectable in the unfertilized egg. One of these proteins is destroyed every time the cells divide. Eggs of the sea urchin Lytechinus pictus and oocytes of the surf clam Spisula solidissima also contain proteins that only start to be made after fertilization and are destroyed at certain points in the cell division cycle. We propose to call these proteins the cyclins.