In this paper the author's object was to show what propelling power was required and produced to push the fœtus through the passages into the world in the easiest deliveries. The fact chiefly founded on, as affording a basis for the necessary calculations, was the occasional birth of the ovum entire, that is, the membranes containing the fœtus and liquor amnii being entire after passing through the maternal passages. In such cases the strength of the amniotic membrane was greater than that of the parturient or expulsive power. The expulsive power was in them never so strong as to break the bag of membranes; and the tensile strength of the membrane being ascertained by experiment, the propelling power could be calculated from it.