. Comparative anatomy. Anatomy, Comparative. 94 COMPARATIVE ANATOMY. forming. Shortly the somites become detached from the lower somatic and visceral sheets of mesoderm and the myocoeles lose continuity with the permanent coelom (Fig. 64B). Eventually, as the somite differentiates, the myocoele is obliterated. The differentiation of the mesoderm at the stage just described— dorsally the pairs of quite detached hollow somites; ventrally the unseg- mented somatic and visceral layers enclosing between them the coelom— has its exact counterpart in Amphioxus. But in Amphioxus segmentation and hollo
. Comparative anatomy. Anatomy, Comparative. 94 COMPARATIVE ANATOMY. forming. Shortly the somites become detached from the lower somatic and visceral sheets of mesoderm and the myocoeles lose continuity with the permanent coelom (Fig. 64B). Eventually, as the somite differentiates, the myocoele is obliterated. The differentiation of the mesoderm at the stage just described— dorsally the pairs of quite detached hollow somites; ventrally the unseg- mented somatic and visceral layers enclosing between them the coelom— has its exact counterpart in Amphioxus. But in Amphioxus segmentation and hollowness of the mesoderm are primary and continuity of the coelomic space is secondarily acquired. In verte- brates segmentation is secondary and so isp also is hollowness except that in cyclo- stomes and some urodele amphibians the early mesoderm appears to have folded outward from the dorsal wall of the enteron and exhibits a thin cavity which Fig. 66.—Stereogram of the may be continuous with the enteric cavity anterior region of an amphibian .p. ^^^ ^j^j^ exceptional Condition, (urodele) embryo, a, anterior end; \ b -> f ar, archenteroh; c, coelom; ch, noto- although it involves uo primary segmen- chordal cells; e, yolk cells; ec,ecto- . ^^ ^g ^he mode of Origin of the derm; mp, mesodermal pouch; ng, . primitive groove; np, neural plate; mesoderm in AmphioxuS. nr, neural folds; sc, segmentation j • f ^^er history, however, the cavity (blastocoele); so, somatic wall -' ' of coelom; 5;^, splanchnic (visceral) differentiation of the vertebrate mesoderm wall of coelom. (From Kingsiey) .^ ^^^^ elaborate than that of Amphioxus, especially in the prospective trunk region. Here a region of mesoderm intermediate between the upper somite or muscle-forming part and the lower peritoneum-forming layers becomes early designated to play a very important role, the formation of the essential structures of the kidneys. In the trunk mesoderm, then, there are upon each side three zones
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