The definitions of germ, embryo, and foetus are purely arbitrary, the new individual being called a germ until the rudimentary characteristics appear; then within a short space of time it is called an embryo, and retains this name for the first three months of gestation, or until the placental circulation is established; then after the formation of the placenta, from the end of the third month to the close of pregnancy, it is called a foetus; then whea it is born and separated from intimate connection with its mother it is called a babe; then while it is dependent on its mother for nourishment, and while it occasionally attaches itself to her breasts, it is called an infant; and subsequently it receives the different titles of child, pubescent, adolescent and adult, until finally it becomes a neuter, to all intents and purposes, with the advance of senile decay.
It is, of course, the same individual throughout all this course.
The reader must not be misled by the scientific phraseology into thinking that the newly created being is anything but human; for this nomenclature has been adopted merely for convenience of description, and is just as artificial as the divisions into which its later life is marked off. Nature has no such lines of demarcation.
The ovum having become a germ-cell as described, its yolk or vitelline substance contracts around the newly formed nucleus which resulted from the blending of the male and female pronuclei, and then the yolh and this new nucleus spontaneously divide into two nucleated spheres, which are simply two new cells, each with a new nucleus, which have been formed by the splitting of the original cell into two halves.
Each of these two new cells subdivides into two other cells; these newly formed cells again subdivide in the same manner, each being the parent, as it were, of a new nucleated cell, and so they continue to subdivide in a geometrical ratio of progression forming 4, 8, 16, 32, and so on. This process is called cleavage, or fission, or segmentation of the cells. In this manner of geometrical increase, rapid multiplication of cells is attained, and the growing child owes its evolution into adult life by the same process of Jissiparous division of cells. This process-called cleavage of the yolk is continued until the whole of the yolk is subdivided into numerous email nucleated cells which form an agglomeration within the zona peUucida looking like a mulberry, from whence it is called the morula stage.
While this morula is developing within the ovum, the latter is at the same time increasing in size by the absorption of albuminous fluid which coats it during its descent along the Fallopian tube.
The cells of the morula, from mutual pressure, become eventually so arranged as to form an envelope, or bladder, which is closely applied all around to the vitelline membrane (zona pellucida). This arrangement of the cells which shows the first indications of coherent tissue ia called the blastoderm.