. Comparative anatomy. Anatomy, Comparative. 84 COMPARATIVE ANATOMY An emliyo of Amphioxus, at a stage when fourteen or fifteen pairs of mesodermal pouches are present, is a delicate colorless transparent animal having a length of about one millimeter and a diameter of one- eighth that except at the somewhat enlarged head end (Fig. 57). It has a straight digestive tube (enteron) /, extending from an anterior mouth to a posterior anus. There is a single gill cleft, opening from the right side of the anterior region of the digestive tube. The mouth also is unsym- metrical at this stage, opening
. Comparative anatomy. Anatomy, Comparative. 84 COMPARATIVE ANATOMY An emliyo of Amphioxus, at a stage when fourteen or fifteen pairs of mesodermal pouches are present, is a delicate colorless transparent animal having a length of about one millimeter and a diameter of one- eighth that except at the somewhat enlarged head end (Fig. 57). It has a straight digestive tube (enteron) /, extending from an anterior mouth to a posterior anus. There is a single gill cleft, opening from the right side of the anterior region of the digestive tube. The mouth also is unsym- metrical at this stage, opening on the left side. Later, as numerous additional gill clefts are formed, they shift their positions so as to become ultimately a series of symmetrically placed paired apertures. Meanwhile the mouth shifts from its original left to a median position. Just above the digestive tube lies the median rod-like notochord {NC) extending the entire length of the animal. Immediately above the notochord is the. Fig. 57.— at beginning of larval period; 14 or 15 pairs of mesodermal somites. Actual length of larva about mm. CG, club-shaped gland; /, intestine; MES, mesodermal somites; NC, notochord; NE, neurenteric canal; NP, neuropore; NT, neural tube; P, pigment spot in neural tube. (After Hatschek.) neural tube (NT), its somewhat enlarged anterior region suggesting a brain. At the anterior end of the neural tube the dorsal neuropore (NP) is still open. The neurenteric canal (NE), at this stage, has ordinarily become closed. In the anterior region, where the differentiation of the mesoderm is most advanced, a coelom intervenes between the enteric tube and the outer body-wall (Fig. 56, C). The body wall (somatopleure) consists of the ectoderm and the somatic layer of mesoderm. The enteric endoderm together with the contiguous visceral or splanchnic layer of mesoderm constitute the wall (splanchnopleure) of the digestive tube. The somatic and visceral sheets of mesoderm provide
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Keywords: ., bookcentury1900, bookpublisherphi, booksubjectanatomycomparative
