192 COMPARATIVE ZOOLOGY. 



Figs. 4r, 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 germinal 

 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 -for- 

 mative and food yolk. The latter is of a more oily nature 

 than the former, and is usually not segmented with the 



