DEVELOPMENT. 197 



usually motile, and in that a large number are usually pro- 

 duced from a single primary reproductive cell of the ani- 

 mal, while the egg represents the entire primary cell. The 

 union of the sperm-cell with the germinal vesicle {^f'ertili- 

 zation) is the first step in development, and without it the 

 egg will not develop. But the nature of the process is 

 unknown. 



CIIAPTEE XX.* 



DEVELOPMENT. 



Development is the evolution of a germ into a com- 

 plete organism. The study of the changes within the egg 

 constitutes the science of Embryology ; the transforma- 

 tions after the egg-life are called metamorphoses, and in- 

 clude growth and repair. 



The process of development is a passage from the gen- 

 eral to the special, from the simple to the complex, from 

 the homogeneous to the heterogeneous, by a series of dif- 

 ferentiations. It brings out first the profounder distinc- 

 tions, and afterwards those more external. That is, the 

 most essential parts appear first. And not only does de- 

 velopment tend to make the several organs of an individ- 

 ual more distinct from one another, but also the individual 

 itself more distinguished from other individuals and from 

 the medium in which it lives. With advancing develop- 

 ment, the animal, as a rnle, acquires a more specific, defi- 

 nite form, and increases in weight and locomotive power. 

 Life is a tendency to individuality. 



The first step in development, after fertilization, is the 

 segmentation of the egg, by a process of self-division. In 

 the simplest form, the whole yolk divides into two parts ; 

 these again divide, making four, eight, sixteen, etc., parts, 



* See Appendix. 



