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. 
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, 
