1. Tissue carbonic anhydrase is usually formed at an early stage in the embryonic development of the chick and mouse. The enzyme does not appear in the blood until a relatively late stage has been reached.
2. In the erythrocytes, it is probable that the enzyme is confined to those cells produced in bone marrow.
3. Evidence is presented to support the theory that towards the end of development, there is a replacement of red cells which contain an embryonic type of haemoglobin but no carbonic anhydrase, by corpuscles in which the enzyme is present together with the adult type of haemoglobin.