A lethal dwarf mutation in the rabbit, which appeared during the course of experiments designed for other purposes, has been the subject of a previous report (1) from this laboratory. These dwarfs, though born alive, all die within a few days. They are delicately formed and appear to be fully developed, but the bones of the calvarium are usually incompletely calcified. The observations on these a~imals appeared to indicate an incomplete recessive mutation which is characterized morphologically by a dwarfing effect and functionally by disorders in both heterozygous and homozygous individuals.The occurrence of this dwarf mt~tation emphasized the desirability of a general consideration of the factors influencing the birth weight of the rabbit. Studies pursued with this end in view had, however, the particular purpose of ascertaining to what extent environmental influences could account for the abnormally low weights of the animals denominated dwarfs. In approaching this problem, it was believed desirable first to determine whether intrauterine factors were operative in producing variations in fetal weight at or near term. An analysis of data accumulated with this end in view, together with certain collateral findings, forms the basis of the present report. Future communications will deal with the influence of gestation period, litter size, age and weight of the doe, season, and breed, on the birth weight of the rabbit. Material and MethodsThe observations on 475 fetuses carried by 71 pregnant rabbits form the basis of the present report. The majority of the does, all of which were bred in the colony, were the progeny resulting from miscellaneous hybrid matings, but a few 901 on

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For several years a colony of standard bred rabbits has been maintained by this laboratory as the basis for the study of problems in constitution. The foundation stock for the colony was built up from pedigreed and registered animals purchased in this country and abroad. The breeds represented were chosen mainly on the basis of physical conformation, and the stock includes a full range of contrasting types from the small Polish to the Giant Flemish, and the cobby Dutch to the racy Belgian. Representative color varieties and coat characters are also included?One of the purposes for which this colony was organized was to provide material of known quality for the study of relations between constitution and susceptibility to disease (1). As an approach to these problems, experiments were carried out with pure bred rabbits of various lines to determine, on the one hand, whether such differences as are represented by breed characters may be associated with distinctive differences in the reaction to disease-producing agents, and on the other, for the purpose of selecting the most suitable material for a more detailed study of any relations which might be found. The present report is concerned with observations on the response of different breeds of rabbits to one of the agents employed, namely, Tr. pallidum.From time to time there has appeared in the literature of experimental syphilis suggestions that resistance to infection may be related to the breed or color of the 1 For a description of American breeds and standards of perfection, the reader is referred to the Guide Book issued periodically by The American Rabbit and Cavy Breeders Association, Inc., 7408 Normal Avenue, Chicago, Ill. Breeds recognized in other countries vary somewhat from those in America, but the standards agree in most essential respects. 907 on