It has been truly said, that “in all the sciences of observation, the great difficulty generally consists in taking the first steps.” A hundred and fifty years have now elapsed since the celebrated Regner de Graaf, after a series of well-conducted observations, maintained that the ovum of the Mammalia must exist already formed in the ovary; an opinion which, after meeting with violent opposition, appears to have been nearly abandoned, and superseded by the notion countenanced by Haller, that the ovum was formed in the Fallopian tube out of a substance discharged from the ovary. A century after De Graaf had promulgated his opinions, Cruikshank arrived at the same conclusion, that the ovum was really formed in the ovary; but he sought it there in vain. Prevost and Dumas in 1824 obtained a glimpse of something that must have been the ovum in that organ; Von Baer in 1827 found and recognised it there. This important discovery of Baer formed an epoch in the history of development; but it was a “first step,” and the object one of extreme minuteness. It was therefore not surprising if the excellent discoverer did not see or justly estimate all that appertained thereto; and he said himself “there remains yet many a thing that will become a prize” for others.