Normal postembryonic and regenerative development of segmented and nonsegmented skeleton rays in fins of Trichogaster sumatranus and in Carassius auratus were studied. In both species the general features of development are the same. In Carassius all fins have a segmented skeleton. Trichogaster has a dorsal and a long ventral fin with nonsegmented bony rays in their anterior, but with segmented bony rays in their posterior, parts. In the tail and pectoral fins the skeleton is segmented. The fins grow in length by adding new material (segmental bony plates or nonsegmented bone in the skeleton) to their distal end and in breadth (at least the tail fin) by adding new bony rays (together with other tissue) at both edges. Both types of bony rays are formed by the so-called “intramembranous ossification.” Yet segmented and nonsegmented bony rays grow in a different manner: (1) The segmental bony plates are preformed by longer strips of closely packed mesenchymal cells (about 2–3 times as long as an average segment). These cell strips ossify in proximodistal direction while growing successively in length and up to a certain degree in thickness by aggregation of more mesenchymal cells. Thus growth in length of the segmented skeleton keeps ahead of ossification as long as the fin is growing. (2) In the nonsegmented bony rays growth and ossification are synchronized. After a few cells have aggregated at the distal part of the already formed bone, they ossify immediately. This goes on continuously and, thus, cellular stages at the distal part of the rays no longer exist. During regeneration the cell supply comes from a blastema at the distal end of the fin. This is probably also true for normal development; however, it is not as obvious as in regeneration. The cell divisions in the blastema of segmented fin skeletons seem to occur in periodically alternating high and low growth rates. In the growth of the blastema in fins with nonsegmented skeleton rays no evidence for this was found. Cell divisions in large numbers occur only in the very distal end of the regeneration blastema. A basement membrane is not clearly visible there in the light microscope and, thus, it is difficult to make a distinction between ectodermal and mesodermal layers. Somewhat more proximally, a normal skin is formed and mesenchymal cells are loosely distributed throughout the inner space. The first steps in aggregation of the mesenchymal cells in order to form a skeleton could be understood simply in terms of statistics. In regeneration joints in the segmented skeleton rays arise in such a way that in these areas small groups of cells move out from the cell strips before they differentiate to bone matrix. Thus, a gap (joint) in the ray arises. Almost all joints formed during the first period of regeneration are more or less incomplete, because ossification sets in early and before formation of the gap is finished. Similar processes seem to occur in normal development near the base of the segmented rays. New segments are added immediately in their final length, which is not constant. In the different parts of the fin distinct average lengths of the segments are exhibited. The rays develop in grooves of the skin, but in proximodistal direction no differentiations in the skin which could occur in connection with segment formation are to be seen. Also, the location of a joint very probably is not directly dependent on the location of other joints. Yet, the assumed growth periodicity of the regeneration blastema seems to be involved in segment formation.