. Chordate morphology. Morphology (Animals); Chordata. entoderm. As the body axis is laid down behind this area, the notochord is separated at the primitive node from the entoderm; laterally and anteriorly, mesodermal plates ex- tend out and later form the somites and enclose the coe- lomic space. The extraembryonic mesoderm in the human appears early by egression of cells from the central mass. In the marsupials there is some variation in eggs. That of the opossum has a distinct albumin layer enclosed by a shell membrane. The egg is isolecithal, the cleavage holo- blastic; the resulting cells
. Chordate morphology. Morphology (Animals); Chordata. entoderm. As the body axis is laid down behind this area, the notochord is separated at the primitive node from the entoderm; laterally and anteriorly, mesodermal plates ex- tend out and later form the somites and enclose the coe- lomic space. The extraembryonic mesoderm in the human appears early by egression of cells from the central mass. In the marsupials there is some variation in eggs. That of the opossum has a distinct albumin layer enclosed by a shell membrane. The egg is isolecithal, the cleavage holo- blastic; the resulting cells tend to orient around an internal blastocoel. Among the placentals there may be some albumin out- side the zona pellucida, or chorion. The egg is isolecithal, centrolecithal, or alecithal and cleaves holoblastically to form a solid ball of cells, the morula. From the morula a blastula is formed by the development of a central hollow (Figure 7-15). The development of this hollow also produces a thick mass of cells at one end. From this central mass the embryo and its overlying membranes develop. In some of the marsupials, a central mass of cells is only poorly devel- oped or even lacking. In the latter case, a central mass is produced by increased mitoses at one end of the blastula. The outer layer of cells of the placental blastula forms a trophoblast. The central mass gives rise to entoderm by in- gression of cells which spread out as a lining layer, convert- ing a part of the blastocoel to an archenteron. The central mass, inside the trophoblast, now becomes an epiblast. In some placentals the trophoblast overlying the central mass disintegrates, whereas in other mammals, the primates, or rat, the trophoblast remains. Development from this point follows two lines. In those with the overlying trophoblast, as amniotic cavity, enclosed by epiblast cells, hollows out above the embryonic disc. In the other group, this cavity develops by a sinking of the ger- minal dis
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