No subject connected with physiology has employed the attention of the anatomist and philosopher than the first formation of the embryo in the class mammalia, and yet even at this day, when the same subject has been completely investigated in oviparous animals, and it is known that an ovum is formed in the ovarium of the quadruped, the circumstances respecting its impregnation have not been ascertained. The great Harvey, although supplied by the munificence of his king with deer in all the different stages after being fit for the male, was unsuccessful. John Hunter, who prosecuted the same enquiry in the ewe, also failed. His brother, Dr. William Hunter, in his splendid work on the Gravid Uterus, has given the most correct representations of the human embryo from the end of the third week till the time of birth, but has not said any thing upon the subject of impregnation.