[Illustration: FIG. 6.—First commencement of the development of a mammal’s egg, the so-called “yolk-cleavage” (propagation of the egg-cell by repeated self-division). _A._ The egg, by the formation of the first furrow, falls into two cells. _B._ These by division fall into four cells. _C._ These latter have fallen into eight cells. _D._ By continued division a globular mass of numerous cells has arisen.] The same process of cell-division now repeats itself several times in succession. In this way, from two cells (Fig. 6 _A_) there arise four (Fig. 6 _B_); from four, eight (Fig. 6 _C_); from eight, sixteen; from these, thirty-two, etc. Each time the division of the kernel-speck precedes that of the kernel; this, again, precedes that of the cell-substance, or protoplasma. As the division of the latter always commences with the formation of a superficial annular _furrow_, or cleft, the whole process is usually called the _furrowing of the egg_, or yolk-cleavage, and the products of it, that is, the cells arising from the continued halving, are called the _cleavage spheres_. However, the whole process is nothing more than a simple, oft-repeated _division of cells_, and the products of it are actual, naked _cells_. Finally, through the continued division or “furrowing” of the mammal’s egg there arises a mulberry-shaped ball, which is composed of a great number of small spheres, naked cells, containing kernels (Fig. 6 _D_). These cells are the materials out of which the body of the young mammal is constructed. Every one of us has once been such a simple mulberry-shaped ball, composed only of small equi-formal cells.