We may now turn our attention to the consideration of some of the phenomena connected with the early processes in the development of the embryo. We may assume that the eggs and sperms have reached such a stage in their life history that they are now mature. All that is necessary in order that the development of an embryo should result is that union of the two elements should take place. Many complicated changes have occurred in the constitution of these eggs and sperms before this stage is reached, but into these we need not enter. It will suffice for our purpose to assume that they are now mature. Then as the result of a natural instinct which suggests certain thoughts and emotions to the male and female animals, which in turn are followed by certain definite acts, the sperm-cell from the male and the egg or ovum-cell from the female are brought into contact. This contact takes place in such circumstances that the united elements are able to be protected and nourished and so, fertilisation having thus occurred, development begins. The characters of these two wonderful cells, which by their union ultimately cause the production of an embryo, are briefly as follows. The element from the male, the sperm that is, is an extremely minute cell which is only about 1/300 of an inch in length. As seen under a high power of the microscope it is composed of two portions which are spoken of as a head and the tail. The former is a flat, oval part, and behind this is the rounded body ending in the long tail which is some four-fifths of the total length. This long tapering tail gives to the sperm its power of movement, for it is supposed that as the result of the rotating or lashing movements of this tail the cell is propelled. Indeed its rate of motion has been actually studied, and estimated to be at about one-eighth of an inch per minute. The cell contributed by the female, the ovum that is, has quite a different structure and microscopical appearance. Compared with most cells it is rather large, almost round in shape, having a diameter of about 1/120 of an inch. Up to the time we are now considering, this cell, along with a great many others like it, has been stored within the female ovary, from which organ an ovum periodically escapes. Unless fertilisation takes place by union with a sperm the discharged ovum perishes. Should, however, the sperm-cell be available, and should it have been able to reach a situation at which fertilisation can take place within, the chain of events which constitute development begins. But before fertilisation can take place the ovum has undergone what is called the process of maturation, in which it divides twice, giving off two small portions of itself in the process. The result of this is that half the number of chromosomes in the ovum are lost. This process of maturation has already taken place in the sperm before it leaves the body of the male.