A certain fold, the head-fold, arises at the front end of the embryo by the bending of the spinal column. Beneath the head-fold arise five processes or gills, as in fishes, which later on are transformed into the face of the fetus. Four other processes are budded off from the trunk and subsequently become the extremities. A furrow at the ventral side of the embryo shows the origin of later trachea and lungs. On both sides of the head-fold can be seen two pits for the eyes. At this point, the embryo is in the same stage of development as many arthropoda. The skeleton begins now to ossify. The heart tube begins to bend and takes the form of an “S.” In this way the tube is turned into an auricle and ventricle as in the amphibia. The ventricle is then divided by a partition as in the reptiles. One part of the nerve-tube is differentiated into three cerebral vesicles, as in birds. Thus the embryo resembles in its structure at different stages the structure of the different types of the animals of the animal kingdom. The different formations do not follow the chronological order as described, but, as a rule, they take place synchronously. At the end of the fourth month the fetus is about sixteen centimeters or six inches long and has reached its definite human shape. CHAPTER II EVOLUTION OF THE GENITAL SYSTEM _A. The Indifferent Stage_ Before the last described stage has been reached, there has developed simultaneously with the other organs a set of organs, known under the name of the genito-urinary system, which deserves here our special attention. The urinary secretion is effected throughout the animal kingdom by three systems: the pronephros or the head-kidney; mesonephros, or primitive kidney, or Wolffian body, “Urniere”; and metanephros, or true kidney. The pronephros must be regarded as the phylogenetically oldest part, since only traces of it are found in the human embryo. Here in the earlier stages of embryonic development, the Wolffian body is the organ for the urinary secretion. The Wolffian body, or mesonephros, appears in the shape of two longitudinal protuberances on either side of the mesentary along the spinal column. The protuberances consist of a series of transverse excretory tubules or nephrides. These tubules open into two pronephric ducts, or Wolffian ducts, which are running alongside the abdominal aorta. These two Wolffian or primitive male ducts open at the caudal end of the embryo into the hind-end of the alimentary canal, or the cloaca (Cut III, Fig. 2).