. Carnegie Institution of Washington publication. 14 YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. same stage of development as the corresponding somite of a chick embryo of the 18-somite stage described by Williams. The dermatome is a flattened plate underlying the ectoderm and formed of the whole dorsolateral wall. The myotome is flexed under the dermatome medially and extends out about half the extent of the dermatome laterally, lying in con- tact with the latter and including between itself and the latter a small cut-off portion of the general cavity of the somite in the dorsomedial
. Carnegie Institution of Washington publication. 14 YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. same stage of development as the corresponding somite of a chick embryo of the 18-somite stage described by Williams. The dermatome is a flattened plate underlying the ectoderm and formed of the whole dorsolateral wall. The myotome is flexed under the dermatome medially and extends out about half the extent of the dermatome laterally, lying in con- tact with the latter and including between itself and the latter a small cut-off portion of the general cavity of the somite in the dorsomedial angle. There is a deep cleft, named by Williams the lower myotomic groove, separating the myotome from the sclerotome below it. The sclerotome is formed of a mass of looser cells forming the ventromedial and ventral part of the wall of the somite, and from the ventral surface of this two long, curved, plate-like processes pass off. The inner one, the noto- chordal process, passes medially between the medullary canal and the dorsal aorta to the side of the notochord. The lateral one, the aortic process, passes ventrally around the lateral surface of the aorta toward the wall of the pharynx, its ventral end being indistinct and hard to differentiate from the surrounding mesoderm. The notochordal process arises from the whole extent of the ventral surface of the sclerotome, but the aortic process comes only from the anterior two-thirds, and there is a distinct interval between it and the next succeeding aortic process, except in the case of the second and third somites, in which these processes are fused. The cavity of this somite is distinct and irregular and is not very large. The core is fused with the sclerotome and is U. M. G. 1. Transverse section through the middle of the fifth mesodermic somite on the right side in Embryo VI. Drawn with camera lucida. Magnification X 270. 2. Transverse section through the middle of the twelfth mesodermic somite on the right si
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Keywords: ., bookauthorcarnegie, bookcentury1900, bookdecade1910, bookyear1915
