THE CLEAVAGE OF THE YOLK. 209 
original cellular membrane, has become two naked cells, 
each possessing its own kernel (Fig. 6). 
Fie. 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, ete. 
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 com- 
mences 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 
