Maturation in vitro of oocytes of the newt, Notophthalmus viridescens, is inducible with progesterone after in vivo treatment of females with gonadotropin; few oocytes mature in vitro in the absence of such gonadotropin treatment. Chromosomes of most large oocytes of animals not receiving gonadotropin are still in the lampbrush condition; chromosomes from gonadotropin-treated animals are shorter and the lateral loops are less profuse and somewhat retracted. The chromosome condition, then, can be correlated with susceptibility to progesterone induction of maturation in vitro. As maturation progresses, the germinal vesicle moves toward the surface and decreases in size, with an apparent loss of nuclear material from the centripetal end. Although lateral loops of most chromosomes disappear during the changes in the germinal vesicle, profuse loops develop during this period at the sphere loci, which were previously devoid of loops.

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