It has proved a difficult task to secure very young human embryos, and many of the ideas we hold relative to the initial stages in the development of man are based upon what has been found true in certain mammals, the class of animals to which we belong. The youngest human ovum known at present has already undergone about two weeks' development, and there the embryo is represented by a flat disk. From this stage to the stage of complete development a satisfactory series of embryos has now been collected, but it is impossible to give here, even in outline, a description of the evolution of the human embryo. No one can understand this intricate subject without the aid of diagrams, models, and other material beyond the reach of all save laboratory workers. By the end of the second month the development of the embryo has advanced so far that anyone could recognize its human shape. About that time, too, the external sexual organs make their appearance. At first these are quite similar in both sexes; and, if they are used as the criterion, it is possible only toward the end of the third month to say whether the embryo is a male or female. THE DETERMINATION OF SEX.--The fact that a number of months pass before the sex can be distinguished by an external examination of the fetus has led to the erroneous belief that it can be influenced during the early part of pregnancy or actually determined at will. Various means to accomplish this have been suggested; many of them depend upon modifying the mother's mode of living according as a boy or girl is desired. The most widely known of these doctrines, that of Schenck, was to the effect that the sex of the offspring is always that of the weaker parent. He suggested, therefore, that increasing the vigor of the mother by an appropriate diet would produce a male child, whereas a decrease in her strength would lead to the opposite result. His views, however, were incorrect. After studying extensive statistics Newcomb came to the conclusion that "it is in the highest degree unlikely that there is any way by which a parent can affect the sex of his or her offspring." Moreover, the results of experimental research clearly indicate that we shall never possess the means by which a mother may control the sex of her child. In the main laboratory investigations have sought to answer two questions. First, at what time is the sex of the offspring determined? and, second, what accounts for the origin of a male in one instance and of a female in another? The study of these problems has been carried on chiefly in connection with insects, worms, and fowl; but as yet insurmountable difficulties have prevented similar investigations in higher animals. For this reason, it is not without the greatest caution that results thus far obtained may be assumed to apply to man.