1. When tissues of hydra are expressed through bolting cloth having meshes as large as those used for sponges and hydroids, large masses of tissue, entire tentacles, entire buds and the like, come through so that this method is not adequate for hydra. 2. When hydras are expressed through cloth of fine mesh (40 micra), detached and fragmented cells, and masses in which entoderm is separated from ectoderm, are produced. Such cells or fragments do not reunite to form a new individual. 3. However, small cut fragments (25 to 60 from the body of an individual) under favorable conditions reunite to form a new hydra. Such fragments carry both entoderm and ectoderm united in the usual way. 4. To determine the rôle of the body layers in the reunition of parts, methods were devised for exposing each of the three layers separately and bringing them each in contact with one another. This gave the following results: Entoderm unites readily with entoderm and also with mesoglea, but does not unite with ectoderm. Mesoglea adheres occasionally to mesoglea, but does not unite with ectoderm. Ectoderm does not fuse with either of the other layers, nor with another layer of ectoderm. 5. In the union of fragments to form a new individual, the initiative is taken by the entodermal cells, which send out protoplasmic processes which interweave with one another. The gaps in the ectodermal layer are seemingly covered by regenerative growth of ectoderm, spreading over the united entodermal masses. This explains why reunition does not occur when entoderm and ectoderm are separated into different fragments. 6. Thus the diversity of ectoderm and entoderm persists through the process of fusion and regeneration. Whether additional production of either or both types from the interstitial cells occurs was not determined. 7. A reunition mass formed by uniting fragments of the green and brown hydra disintegrates. Or if a small amount of tissue from one species is united with a large amount from the other, fusion occurs, but the kind of tissue that is present in small amount later sloughs off. 8. The fragments first unite to form a plate with ectoderm on one side, entoderm on the other. The edges then roll together to form a cylinder, with entoderm within. Head and foot are then produced. 9. To determine the fate of differentiated parts, the foot of the original hydra was stained with methylene blue, so that the fate of the fragment which it constitutes could be followed. The original foot retains its identity and function, later becoming the foot of the individual regenerated from the fragments. But accessory feet may also be produced from other tissues. 10. Many irregularities and abnormalities are produced in the early stages of regeneration, probably as a result of incomplete fusion in some regions. Supernumerary heads, feet, and tentacles are common, and form and structure are frequently abnormal. 11. These abnormalities are later regulated by: splitting of parts, absorption of parts, and migration and coalescence of parts. The two former methods are unusual, as most regulation involves migration and coalescence. Only like organs coalesce.