THE LAW OF EVOLUTION 



glossy ; the cells composing the lower stratum 

 become- less and less crowded, until the result is 

 a spongy surface, filled with innumerable pores, 

 through which the moisture of the plant may be 

 exhaled. Finally a differentiation arises between 

 the axillary buds, some of which elongate into 

 branches, repeating the chief characteristics of 

 the stem, while others are developed under the 

 still more heterogeneous forms of flowers, with 

 their variously cleft calyx and corolla, and their 

 variously compounded stamens and pistils. 



In the fertilized mammalian ovum the ear- 

 liest step toward heterogeneity consists in the 

 division and redivision of the nucleated embry- 

 onic cell. As the cell-nucleus grows, by con- 

 tinuous integration of the nutritious protoplasm 

 in which it is embedded, it slowly becomes 

 grooved, and ultimately divides into a pair of 

 nuclei, about each of which is formed a cell-wall. 

 This process continues until the entire yolk is 

 absorbed, by which time it has become differen- 

 tiated into a mulberry-like mass of cells. And 

 these cells, at first all alike spherical or nearly so, 

 become club-shaped or hexagonal or pointed, 

 as the mass further consolidates and squeezes 

 them together. A grand differentiation next 

 occurs between the outer and inner portions of 

 the yolk-mass : the outer cells become flattened 

 and pressed together, so as somewhat to re- 

 semble a mosaic pavement, and thus form a 



vol. n 225 



