. Analysis of development. Embryology; Embryology. Fig. 73. Left: Outline of normally invaginating gastrula. a, The originally active zone derived from the central spot of the gastral plate, and p from the plastic zone of the plate, which formed the periphery of the plate surrounding a; and r, the new active ring. Right: Diagram showing form of cells in neurulation. In the deformation which makes the furrow, the individual cells (one shown isolated) have the form of truncated wedges, two sides of which are inclined to each other. (Figures from A. R. Moore, '41.) physicochemical causes of epibo
. Analysis of development. Embryology; Embryology. Fig. 73. Left: Outline of normally invaginating gastrula. a, The originally active zone derived from the central spot of the gastral plate, and p from the plastic zone of the plate, which formed the periphery of the plate surrounding a; and r, the new active ring. Right: Diagram showing form of cells in neurulation. In the deformation which makes the furrow, the individual cells (one shown isolated) have the form of truncated wedges, two sides of which are inclined to each other. (Figures from A. R. Moore, '41.) physicochemical causes of epiboly or de- lamination. However, since the invagination of the gastrula is a process upon which all considerations of tissue foldings could be based, to learn how vegetal cells undergo this invagination might supply the key to similar processes (such as neurulation) in the formation of other organs (see Fig. 73). One possible approach to a study of the mechanism of gastrulation might be assumed to lie in the fact that the inpocketing of the gastrula of the sea urchin (also of Am- phibia) can be reversed into an out-pocket- ing or evagination. Such evaginated embryos, called exogastrulae, were produced by Herbst (1892) by treatment of the eggs during early segmentation with a small amount of lith- ium salts added to sea water. The lithium ion itself is nonspecific, since exogastrulation can be produced also by a variety of other agents, which so far as can be seen have nothing in common with the lithium ion. However, there has been, as yet, no satis- factory physicochemical explanation of the this hypothesis, it is difficult to explain why the thicker vegetal cells should first invagi- nate. It has been noted also that invagination could take place without increase in cell number. (2) Rhumbler ('02) was probably the first to suggest that in-folding resulted from a change in shape of the vegetal cells. Biitschli ('15) and Spek ('20) postulated a differential swelling between the in
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Keywords: ., bookcentury1900, bookpublisherphiladelphi, booksubjectembryology
