. Bonner zoologische Monographien. Zoology. 31 ning with the segment of Hatschek's nephridium (see below; Stach & Eisler 1998) suggests that this muscle originates in the ventral mesoderm of this segment. Ontogeny of the so-called "muscle tails"' (Fig. 14) The medial group of mesodermal cells that borders the notochord is distinguisha- ble from early larv al stages on. Although the cellular specialization is obviously comparable to that of the lateral mesodermal cell, which underlie the lateral epi- dermis, the shape of the cells in cross sections and horizontal sections differs
. Bonner zoologische Monographien. Zoology. 31 ning with the segment of Hatschek's nephridium (see below; Stach & Eisler 1998) suggests that this muscle originates in the ventral mesoderm of this segment. Ontogeny of the so-called "muscle tails"' (Fig. 14) The medial group of mesodermal cells that borders the notochord is distinguisha- ble from early larv al stages on. Although the cellular specialization is obviously comparable to that of the lateral mesodermal cell, which underlie the lateral epi- dermis, the shape of the cells in cross sections and horizontal sections differs remarkably. Even in the neurula stages with ten mesodermal segments the medial cells show a club-shaped appearance in cross sectional aspect, whereas the lateral cells appear to be rectangular (; Stach 1994). The more narrow dorsal end of the elongated medial cells is already situated at the lateral ventral border of the neural plate (). Rather than an outgrowth of processes from the prospec- tive myocytes, which contact the central nervous system, the contact of the myo- cytes is established early on during the process of mesoderm formation and retai- ned during further de\ elopment. The situation found in the adult with a clearly confined narrow area of contact with the central nervous system in the so-called central motor end plate (Flood 1968) is a result of the changes in overall cell shape of the medial mesodermal cells during myogenesis (Fig. 14). Fig. 14: Schematic line drawings of the trunk mesoderm of developmental stages of Branchiostoma lanceolatum; not to same scale. Two different cell types are represented: a medial group of (prospective) myocytes and a lateral group of (prospective) mesothelial cells. One cell of the medial group is depicted in greater detail to demonstrate the develop- ment of myofilaments and the early association with the ner\'ous svstem. A - Earlv neum- lastage (22hpf. 18°C). B - Neurula stage (35h pf. 18°C). C - Larval stage
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