192 
COMPARATIVE ZOOLOGY. 
Figs. 4, 6, 9-13. This form of reproduction is, naturally, 
confined to animals whose tissues and organs are simple, 
and so can easily bear division, or whose parts are so ar¬ 
ranged as to be easily separable without serious injury. 
The process is most common in Protozoa, Worms, and 
Polyps. 
Budding is separated by no sharp line from Self-divi¬ 
sion. While in the latter a part of the organs of the par¬ 
ent go to the offspring, in the former one or more cells 
of the original animal begin to develop and multiply so 
as to grow into a new animal like the parent. The proc¬ 
ess in animals is quite akin to the same operation in 
plants. The buds may remain permanently attached to 
the parent-stock, thus making a colony, as in Corals and 
Bryozoa ( continuous budding ), or they may be detached 
at some stage of growth {discontinuous budding ). This 
separation may occur when the bud is grown up, as in 
Hydra (Fig. 191), or as in Plant-lice, Daphnias (Fig. 255), 
and among other animals the buds may be internal, and 
detached when entirely undeveloped and externally re¬ 
sembling an egg. They differ, however, entirely from a 
true egg in developing directly, without fertilization. 
Sexual Reproduction requires cells of two kinds, usu¬ 
ally from different animals. These are the germ-cell or 
egg, and the sperm-cell. The embryo is developed from 
the union of the two cells. 107 
The egg consists essentially of three parts, the germmal 
vesicle , the yolk , and the vitelline membrane , which sur¬ 
rounds both the first. It is ordinarily globular in shape. 
Of the three parts, the primary one is the germinal vesi¬ 
cle—a particle of protoplasm. The yolk serves as food 
for this, and the membrane protects both. When a great 
mass of yolk is present, it is divisible into two parts—/w- 
mative and food yolk. The latter is of a more oily nature 
than the former, and is usually not segmented with the 
