Development of the abdominal musculature of Hynobius nebulosus

Development of the abdominal musculature of Hynobius nebulosus was studied by dissection under binocular magnifying glass and by serial sections, partly stained with some his tochemical methods, the emphasis being laid oa the origin of the transverse abdominal muscle. Differentiation of the dorsal trunk musculature derived from myotomes is performed in "the dorsal Growth-Zone", and this form of muscle-differentiation is common to that of the ven tral trunc musculature derived from the ventral myotome processes. The internal oblique mescle and the external oblique muscle are derived respectively from the inner and the external la yer of "the ventral Growth-Zone" which appears at the ventrolateral region of the early developed ventral trunk musculature. The former grows ventrally, the latter dorsally from the ventral end of the former. The rectus abdominis muscle is also formed from the veatral end of the "Growth-Zone", and grows medially from the ventral end of both oblique muscles. After the above mentioned muscles have completely differentiated, "the sabvertebral Growth-Zone" appears in the subvertebral triangular region between Chorda and the ventro nedial part of the internal oblique muscle. In this region the subvertebral muscle is formed. At the same time the transverse abdominal muscle d in the narrow space between the internal oblique muscle and Peritoneum, but it is difficult to decide whether the Growth-Zone from which the transverse abdominal muscle is derived belongs to the internal oblique muscle or Peritoneum as the space is very narrow. It mav ba sure that the taaisverse abdominal muscle has closely relation to the internal oblique muscle as both muscles differentiate at the same time. The rectus lateralis muscle dsvelops in "the Growth-Zone in the lateral line" and remaining superficial muscles of trunk also develop superficially on the precedingly differentiated trunk musculature by the same way as above mentioned muscle-differentiation.