An hereditary type of dwarfism in the rabbit has been described. In contrast to the dwarfs described in other animals, this type is evident at birth and conforms to the classification, nannosomia primordialis, as used in human pathology. In homozygous form the variation is lethal and produces a miniature individual approximately one-third the size of its normal sibs. Heterozygous animals are approximately two-thirds the size of normal sibs at birth and never attain an equal stature.

The expression of the variation is modified by genetic factors carried by a line of cretinoid animals and, rarely, dwarfs derived from crosses with this line survive for 1 to 2 months. The striking changes in such survivors are hypertrophy and hyperplasia of the acidophilic cells of the pituitary and atrophy of the gonads. Such changes are not present in ordinary dwarfs and it is concluded that the acidophilic hyperplasia represents the influence of the modifying factors of the cretinoid line and supplies the growth hormone responsible for survival. The gonadotropic hormone is not supplied by the secretory activity of these cells and as a result the gonads atrophy.

The evidence at hand indicates that the primary effect of the dwarfing gene is an inhibition of the secretory functions of the pituitary. In homozygous individuals, the inhibition is complete and the variation is expressed as a lethal dwarf. In heterozygous animals, the function of the organ is altered, producing an undersized individual. The modifying factors of the cretinoid line act either to partially remove the inhibition or to alter the constitution of the animal so that life is possible for a short period without the full complement of pituitary hormones.

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