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 {fertili¬ 
zation) is the first step in development, and without it the 
egg will not develop. But the nature of the process is 
unknown. 
CHAPTER 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, 
* See Appendix. 
