#DORSAL LIP OF THE BLASTOPORE SKIN#
Since the results of induction are different for different organ rudiments, it must be presumed that there exist inducing substances with specific action, at least to a certain extent thus, the lateral mesoderm induces differentiation of the skin but not neural plate from the very same kind of ectoderm. Further examples are mentioned below in connection with development of the various organs. The notochord is a source of induction for the development of the adjoining somites and nephrotomes the latter appear jointly to induce development of limb rudiments from the lateral plate mesoderm. Induction is responsible not only for the subdivision of ectoderm into neural plate and epidermis but also for the development of a large number of organ rudiments in vertebrates. Inducing substances are active on vertebrates belonging to many different classes e.g., inductions of primary organs have been obtained by transplanting mammalian tissues into frog embryos or by transplanting tissues of a chick embryo into the embryo of a rabbit. The inducing substance of the mesoderm is a large molecule, probably a protein or a nucleoprotein, which presumably penetrates reacting cells, though direct and unequivocal proof of such penetration is still unavailable. The inducing influence-whatever it might be-is a diffusible substance emitted by the activating cells (the inductor). Though induction requires that the interacting parts come into close proximity, actual contact is not necessary. The influence exercised by parts of the embryo, which causes groups of cells to proceed along a particular path of development, is called embryonic induction. Lateral mesoderm causes overlying ectoderm to differentiate as skin. The dorsal mesoderm, which later differentiates into notochord, prechordal mesoderm, and somites, causes the overlying ectoderm to differentiate as neural plate. Experiments have shown that, at the start of gastrulation, ectoderm is incapable of progressive development of any kind that only after invagination, with chordamesoderm lying directly underneath it, does ectoderm acquire the ability for progressive development. In the development of vertebrates in particular, the sliding of cells (presumptive mesoderm) into the interior and their placement on the dorsal side of the archenteron (in the archenteric “roof”), in immediate contact with the overlying ectoderm, is of major importance in development and subsequent differentiation. Groups of cells that were distant from each other in the blastula come into close contact, which increases possibilities for interaction between materials of different origin. The organization of the embryo as a whole appears to be determined to a large extent during gastrulation, by which process different regions of the blastoderm are displaced and brought into new spatial relationships to each other.
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#DORSAL LIP OF THE BLASTOPORE HOW TO#
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