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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

The pathological histories of two types of familial mammary cancer in the rabbit have been described. One type was distinguished by characteristic antecedent mammary changes similar to those found in Schimmelbusch's disease in women and by a distinctive papillary structure. The second type originated in normal breast tissue and was characterized histologically by an atypical proliferation of acini.

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A series of experiments was undertaken in an attempt to transplant rabbit tumors to animals of alien species using the anterior chamber of the eye as an inoculation site. The uterine tumor H-31 and the breast tumors T-36 and B-240 were successfully transplanted to all the various animals tried, including guinea pigs, swine, goats, and sheep, and the H-31 and T-36 tumors were maintained by serial transfer in the two first species. On the other hand, all attempts to transfer the Brown-Pearce tumor to guinea pigs were unsuccessful. The growth characteristics of the transplanted tumors were generally similar to those observed in the natural host, but noteworthy exceptions occurred. The tumors obtained a blood supply from the foreign host and invaded the periorbital tissues but did not metastasize. Histologically, the cellular morphology of the rabbit tumors was retained, but variations in parenchymal-stromal relations characterized the form of growth in different species.