The neural blastema is induced to separate from the ectoderm; the self-organization of its archencephalic region leads to the segregation of the optic blastema, which acquires the character of a field again and lays out the basic plan of the optic cup. The optic cup, guided continuously by the complementary induction of the epidermis, organizes the lens. The mature nervous system and the mature eye include, in addition to various cell types of ectodermal origin, tissue from the mesoderm. This is incorporated during organogenesis; but the structural layout is determined by the ectodermal blastema alone. With endodermal organs, however, the relationship is different. The determination of the histological differentiation (histogenetic determination) of the presumptive endoderm is completed in the early gastrula (p. 230), but the normal form of the organ cannot be determined by the groups of endoderm cells alone. If presumptive endoderm cells from the vegetal floor of a blastula or young gastrula are isolated in Holtfreter’s solution, the shapeless mass forms a smooth, solid ball within an hour. When such endodermal spheres contact one another, they fuse. During this fusion the endodermal cells exhibit gliding and rolling amoeboid movements. Some twenty hours after isolation the behavior of the endoderm cells changes: the entire cell mass flattens, and its margins spread in all directions over the substratum like a membrane. At the periphery cells may come loose and move a short distance further. The motility of the cells is now different. The cells are flattened and push along with more or less pointed pseudopodia. This change in behavior is autonomous and comes with the passing of time. The tendency for flattening and spreading begins at the developmental stage in which throughout the embryo endoderm spreads along the mesodermal mantle until its edges meet dorsally (Figs. 270b, 298c). In the embryo the spatial arrangement of the flat endodermal layer leads to the formation of the tubular gut. If pure ectodermal explants are maintained for fifteen to twenty days longer, the membrane spread upon the glass differentiates into a single layer of epithelium characteristic of the particular determined region (Fig. 301 a, b), while the central mass generally remains compact but also acquires the cell structure of the gut.
KeywordsLateral Plate Mesoderm Morphogenetic Field Proximodistal Axis Late Neurula Presumptive Endoderm
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