192 COMPARATIVE ZOOLOGY. 



figs. 4, 6, 9-13. Tliis form of reproduction is, uaturally, 

 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 ma}' 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 iudding). 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.'" 



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 membraiie protects both. When a great 

 mass of yolk is present, it is divisible into two paj-ts— ^/b?'- 

 mative &r\d food yolk. The latter is of a more oily nature 

 than the former, and is usually not segmented with the 



