. The chordates. Chordata. Reproduction 261. VMES 1-6' Fig. 214. Arnphioxus. Frontal (horizontal) section of an embryo having six pairs of mesodermal somites. The section is through the notochord and just below the blastopore. At the posterior end of the section may be seen a region where the notochord. endoderm, and mesoderm merge indistinguishably. (A) Archenteron near the blastopore; (EC) ectoderm; (EN) endoderm; (MES 1-6) mesodermal somites; (NC) notochord. (After Cerfontaine. Courtesy, Neal and Rand: "Chor- date Anatomy," Philadelphia, The Rlakiston Company.) material of the ori
. The chordates. Chordata. Reproduction 261. VMES 1-6' Fig. 214. Arnphioxus. Frontal (horizontal) section of an embryo having six pairs of mesodermal somites. The section is through the notochord and just below the blastopore. At the posterior end of the section may be seen a region where the notochord. endoderm, and mesoderm merge indistinguishably. (A) Archenteron near the blastopore; (EC) ectoderm; (EN) endoderm; (MES 1-6) mesodermal somites; (NC) notochord. (After Cerfontaine. Courtesy, Neal and Rand: "Chor- date Anatomy," Philadelphia, The Rlakiston Company.) material of the original mesodermal folds is completely utilized. Later the series of segments is extended backward by addition of successive solid blocks of cells which become detached from the growth zone (germ-ring) encircling the blastopore (Fig. 214). By this means, the number of pairs of mesodermal segments is increased to the adult total, usually 61. In Amphibians. In amphibians, as in Arnphioxus, the blastoporal rim or germ-ring is the all-important source of mesoderm. The am- phibian, however, gives little evidence of anything comparable to the paired mesodermal pouches which push out from the dorsolateral en- doderm of Arnphioxus. During the process of gastrulation in the amphibian, the material destined to become mesoderm lies within the advancing edge of the overgrowing fold (Fig. 211) which is the chief agency in the enclosing of the yolk. As the edge of this fold, the narrowing blastoporal rim, advances, it (in effect) leaves behind it—"behind" being anterior be- cause the fold advances posteriorly—a trail of potential mesoderm which, however, is at first in no way distinguishable from other mate- rial destined to be permanently endoderm (Fig. 211C, EN). That is, the two materials together and in no way delimited from one another constitute the deeper layer of the overgrowing fold. Later this layer virtually splits (the process called "delamination") to form
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