The Cytological Mechanism of the Triploidy-Inducing Effect of Heat on Eggs of the Newt, Triturus Viridescens

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Abstract
Exposure of freshly fertilized eggs of the newt, T. viridescens, to 36[degree] C for 10 min. induces triploidy in over 50% of the surviving embryos. Cytological study of 45 treated eggs fixed during the 1st 5.5 hrs. after treatment confirms the hypothesis that this is accomplished through suppression of the 2d polar body formation which, normally, is not completed until about 1 hr. following fertilization. 22 eggs give evidence of a submergence of the 2d maturation spindle below the egg surface. In the majority of these eggs, the separation of the chromosomes during anaphase and telophase proceeds normally, resulting in the formation of 2 haploid egg nuclei. Later on, both egg nuclei unite with the sperm nucleus while the sperm aster divides in normal fashion to furnish the poles for the 1st cleavage spindle. During anaphase of the 1st cleavage mitosis, 33 chromosomes, the triploid number, may be counted at the poles of the division figure. The principal effect of the heat treatment thus appears to be dislocation of the 2d maturation spindle from the egg surface, rather than an inhibition of the anaphase movements of the chromosomes leading to the formation of a diploid restitution nucleus, as had previously been postulated. However, observations of 3 of the 22 eggs indicate that failure of the spindle mechanism may be superimposed on the submergence of the mitotic figure.