As the apes approach nearest to man of all the mammals in point of structure, we shall expect to hear the same of their vital functions; and that we find to be the case. Everybody knows how closely the habits, the movements, the sense activity, the mental life, and the parental customs of apes resemble those of man. Scientific physiology proves the same significant resemblance in other less familiar processes, particularly in the working of the heart, the division of the breasts, and the sexual life. In the latter connection it is especially noteworthy that the mature females of many kinds of apes suffer a periodical discharge of blood from the womb, which corresponds to the menstruation of the human female. The secretion of the milk in the glands and the suctorial process also take place in the female ape in precisely the same fashion as in women. Finally, it is of especial interest that the speech of apes seems on physiological comparison to be a stage in the formation of articulate human speech. Among living apes there is an Indian species which is musical; the _hylobates syndactylus_ sings a full octave in perfectly pure, harmonious half-tones. No impartial philologist can hesitate any longer to admit that our elaborate rational language has been slowly and gradually developed out of the imperfect speech of our Pliocene simian ancestors. CHAPTER IV OUR EMBRYONIC DEVELOPMENT The Older Embryology--The Theory of Preformation--The Theory of Scatulation: Haller and Leibnitz--The Theory of Epigenesis: C. F. Wolff--The Theory of Germinal Layers: Carl Ernst Baer--Discovery of the Human Ovum: Remak, Kölliker--The Egg-Cell and the Sperm-Cell--The Theory of the Gastræa--Protozoa and Metazoa--The Ova and the Spermatozoa: Oscar Hertwig--Conception--Embryonic Development in Man--Uniformity of the Vertebrate Embryo--The Germinal Membranes in Man--The Amnion, the Serolemma, and the Allantois--The Formation of the Placenta and the "After-Birth"--The _Decidua_ and the _Funiculus Umbilicalis_--The Discoid Placenta of Man and the Ape