Cell death accompanies a variety of developmental processes in many tissues and systems of organs. A comprehensive review by Glücksmann (1951) has revealed how widespread is the occurrence of degenerating cells in vertebrate embryos. Within the developing nervous system, the earliest descriptions of cellular degeneration are due to Barbieri (1905) in Amphibia, and to Collin (1906) in the chick embryo. Since that time, destruction of cells has been observed in the morphogenesis of the neural tube, most recently by Boyd (1955) and by Källén (1955) in early mammalian embryos; in the establishment of regional differences between limb and non limb-levels in spinal ganglia of the chick (Hamburger & Levi-Montalcini, 1949), and also in the histogenesis of Anuran ventral horn cells (Xenopus, Hughes & Tschumi, 1958; Eleutherodactylus, Hughes, 1959).

The present study is also concerned with ventral horn cells in Xenopus. Here the aim has been to draw up a cell balance-sheet during development based both on counts of numbers of ventral horn neuroblasts throughout their differentiation, and also of the number of degenerating cells among them at each stage.

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