THE CLEAVAGE OF THE YOLK. 299 



original cellular membrane, lias become two naked cells, 

 each possessing its own kernel (Fig. 6). 



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. G 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 com- 

 mences with the formation of a superficial annular /it^TOit;, 

 or cleft, the whole process is usually called the furroiving 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 



