Abstract 1. 1. Macronuclear development in E. woodruffi following conjugation takes place in two distinct stages. During the first 18 to 20 h the anlagen is spherical and only later assumes “T” shaped structure characteristic of the vegetative cells. 2. 2. DNA synthesis in the spherical anlagen is a slow and continuous process with no evidence for the type localized synthesis that occurs in the “T” shaped anlagen. In the “T” shaped anlagen DNA synthesis is restricted to small regions which resemble the replication bands of the vegetative cells. 3. 3. In the anlagen, multiple rounds of DNA synthesis take place, each round occupying 45 to 90 min. The anlagen appears to undergo 20 to 24 rounds of DNA synthesis before the commencement of the first division. 4. 4. Breakdown products of DNA of the old macronucleus are reutilized in the development of the new macronucleus. 5. 5. RNA synthesis and accumulation of protein label is a continuous process occurring throughout the anlagen and fragments of the old macronucleus at all stages of exconjugant development.