One of the striking features of this time period is the growth of the tail, which extends to give the embryo its characteristic shape. In addition to tail elongation, the major organ systems, including the somites, the pronephros, the heart, and the central nervous system, form. Segmentation is seen in the forming hindbrain and in the mesodermally derived somites (Fig. 2G). The number of formed somites is a good check on the exact stage that the embryo has reached. Eventually there will be over 30, but, for example, 20 have formed by 19 h of development (Fig. 2H). Spontaneous contractions of the somites begin at the 17 somite stage as the axons from the primary motor neurons first reach the differentiating muscle fibers. Within the central nervous system, 10 distinctive brain regions or neuromeres are evident by 18 h (18 somites). The most anterior of these is the telencephalon, which lies adjacent to the dien-cephalon. These two regions form the forebrain. The midbrain or mesencephalon separates these from the more clearly segmented hindbrain or rhombencephalon. The hindbrain segments are termed rhombomeres of which seven clearly form in the zebrafish.
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