At what stage of its progress from the ovarium through the Fallopian tubes to the cavity of the uterus the ovum meets the sperm-cell from the seminal fluid is not known. Most probably the point of contact is purely accidental. The spermatozoa are capable of rapid movements, and may meet the advancing ovum at any point of its course, even from the moment of its exit from the Graafian cell. Wherever the contact between the germ-cell and the sperm-cell occurs, the resulting changes are wonderful and also inexplicable.

First it is evident that active developmental powers exert themselves, and effect in the contents of the ovum remarkable structural changes. Next it may be predicated that the male spermatic fluid imparts certain qualities and characters to the germ, such as form, constitution, and disposition, which belong to the male, as it later becomes evident that the female parent also shares the power of transmitting these qualities in varying proportion.

Theoretically, it may be considered that the male transmits form, and the female disposition and character, and in man the intellectual power. To this rule there are, however, many very marked exceptions.

After the disappearance of the germinal vesicle, curious changes in the yolk are perceived, resulting in segmentation. First, depressions or notches are noticed in the membrane surrounding the yolk at two points, and these slowly advance through the mass, cutting it in halves, while almost at the same time a similar process is going on in each half, making four divisions, which are divided again and again, until a mulberry mass is formed. This process of multiplication by division of the mass possesses a remarkable significance, which will be referred to in connection with the process of generation in the lower forms of life (fig. 538).

Completion of the process of segmentation leaves the yolk a mass of delicate granular spherical masses, each with a clear centre. Conversion of these masses into cells is effected by the development of an investing membrane round each mass. As soon as the cell - formation is perfected, the peripheral cells arrange themselves on the surface of the yolk, the central masses follow, and finally complete the construction of a thick membrane, which is known as the germinal or blastodermic membrane, which soon divides into two layers; the upper one nearest to the original investure of the yolk, the vitelline membrane, is called the serous layer of the blastodermic membrane, and the lower one the mucous layer. From the upper or serous layer, the outer portions of the animal body, the bones, muscles, and skin, are developed, while the inner or mucous layer, which is in contact with the yolk, forms the internal organs or viscera.