DEVELOPMENT. 197 



usually motile, and in that a large number are usually 

 produced from a single cell of the animal, while the egg 

 represents an entire cell. The union of the sperm-cell 

 with the germinal vesicle (fertilization) is the first step 

 in development, and without it the egg will not develop. 

 But the nature of the process is unknown. 



CHAPTEK 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 rule, 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, 



