THE CELL AND PROTOPLASM. 63 



equally attested each view. It was a Scotchman, 

 Dr. Barry, who finally produced evidence which 

 settled the question from the study of the devel- 

 oping egg. 



The essence of his discovery was as follows: 

 The ovum of an animal is a single cell, and when 

 it begins to develop into an embryo it first simply 

 divides into two halves, producing two cells (Fig. 

 8, a and b). Each of these in turn divides, giving 

 four, and by repeated divisions of this kind there 

 arises a solid mass of smaller cells (Fig. 8, b to/,) 



FIG. 8. Successive stages in the division of the developing egg. 



called the mulberry stage, from its resemblance to 

 a berry. This is, of course, simply a mass of cells, 

 each derived by division from the original. As the 

 cells increase in number, the mass also increases 

 in size by the absorption of nutriment, and the 

 cells continue dividing until the mass contains 

 thousands of cells. Meantime the body of the 

 animal is formed out of these cells, and when it is 

 adult it consists of millions of cells, all of which 

 have been derived by division from the original 



