The breeding season of the opossum (Didelphis virginiana) and the rate of intrauterine and postnatal development

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Abstract
The breeding season of the opossum at Austin, Texas, begins in January, following a three months' anoestrous period. The modal point for ovulation days is reached in the third week. The rate of intra‐uterine development was investigated chiefly by surgical removal of one uterus, noting the stage attained by the ova therein and allowing the surviving uterus to incubate its ova a precalculated period of time. Unique charts epitomize the results. The primitive‐streak stage is completed, the medullary groove and chorda begin at seven and one‐half days post coitum, seven days post ovulationem, leaving only five and one‐half days' actual development of the embryo to birth. The rate of development is compared with Eutherian mammals.The curve of postnatal growth has the shape of embryonic growth curve of higher mammals. The eyes and lips open, at about fifty days (young the size of mice). At this time the young leave the teat for the first time, but are not weaned for about thirty days more. Soon after weaning, the mother may become pregnant again. At ninety to one hundred days (young size of large rats), the young may begin to shift for themselves.