Development of the lateral line system in Xenopus laevis. III. Development of the supraorbital system in triploid embryos and larvae.

During normal development of the supraorbital lateral line system of Xenopus, an elongated streak of primordial cells becomes subdivided into a linear series of cell groups containing only about eight cells each, thus forming a row of primary lateral line organs (Winklbauer & Hausen, 1983a,b). In triploid Xenopus embryos, cell size is 1.5 X normal. When the formation of lateral line organs occurs in triploid primordia, the nascent organs contain only about five or six cells each, i.e. about two thirds of normal. Thus, the increase in cell size is compensated for by a corresponding reduction in cell number, keeping constant the organ size in terms of total cell mass or volume. This result excludes a cell counting mechanism for determining organ size. In diploids, the primary organs, although being of equal size initially, differ vastly in their final size and exhibit a peculiar frequency distribution of organ sizes. A detailed quantitative model for supraorbital lateral line development has been proposed, which accounts for this characteristic frequency distribution (Winklbauer & Hausen, 1983b). This model makes precise predictions as to the frequency distribution of the final size of triploid lateral line organs, where the initial organ size is reduced to five or six cells. These predictions were verified experimentally.