. Biology; the story of living things. REPRODUCTION AND LIFE CYCLES 419 Thus far two germ layers can be differentiated, an outer layer of ectoderm and an inner one of endoderm which lines the archenteron, while the dimin- ishing remains of the blasto- coel lie between. This stage is suggestive of those organisms, like the coelenterates, which characteristically possess only two germ layers even in the adult condition, and are there- fore designated as diplohlastic. Mesoderm Formation The details of the further development of the embryo vary considerably, depending upon the form studied, but al
. Biology; the story of living things. REPRODUCTION AND LIFE CYCLES 419 Thus far two germ layers can be differentiated, an outer layer of ectoderm and an inner one of endoderm which lines the archenteron, while the dimin- ishing remains of the blasto- coel lie between. This stage is suggestive of those organisms, like the coelenterates, which characteristically possess only two germ layers even in the adult condition, and are there- fore designated as diplohlastic. Mesoderm Formation The details of the further development of the embryo vary considerably, depending upon the form studied, but all of the higher forms above the coelenterates produce a third germ layer called the mesodcrjyi. The elaboration of mesodermal tissue may come from either, or possibly both, of the primary germ layers. In all of the vertebrates, two sheets of mesodermal cells are formed, an inner splanchnic layer associated with the inner tube, or developing gut, and an outer so7natic layer, which is contiguous with the ectoderm. Loosely scattered mesodermal cells {mesenchyme cells), de- rived from these more compact layers, fill in the narrow spaces between the gut and splanch- nic layer and between the somatic layer and mccUxllarx plate .Tn€.ciunary fold iwtochortt mesoderm endoderm cocnal ocnd, tijcbe. myotorne rnyocoel dermatome. fn<as«ntery Somatic and. splanchnic mesooLerm ©ncCocC<sr-rn .<SCtodernri Diagram of a generalized vertebrate to show the origin and early differentiation of the ectoderm, endoderm. and mesoderm. (I) shows the mesoderm arising by means of the enterocoelic pouches budfling off from the archenteron. Above and between these pouches lie the beginnings of the notochord. In (II) the medullary plate has formed the neural tube and the mesoderm has become differentiated into regions which will form somites (myotomes), kid- neys, and linings of the body cavity. This differentiation goes still further in (III). (After McEwen.). Please
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