REPRODUCTION 



403 



more cells of the original animal begin to develop and 

 multiply so as to grow into a new animal like the 

 parent. The process in animals is quite akin to the 

 same operation in plants. The buds may remain per- 

 manently 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 (dis- 

 continuous budding). This separation may occur when 

 the bud is grown up, as in hydra (Fig. 18), or as in plant 

 lice, daphnias (Fig. 56), and among other animals the 

 buds may be internal, becoming detached when entirely 

 undeveloped and externally resembling an egg. They 

 differ, however, entirely from a true egg in developing 

 directly, without fertilization. 



Sexual Reproduction requires cells of two kinds, 

 usually from different animals. These are the germ 

 cell or egg, and the sperm cell. The embryo is devel- 

 oped from the cells which are formed by the repeated 

 divisions of the ovum which take place as a result of its 

 union with the sperm cell. 161 



The egg consists essentially of three parts, the 

 germinal vesicle, the yolk, and the vitelline membrane, 

 which surrounds both the first. It is ordinarily globular 

 in shape. Of the three parts, the primary one is the 

 germinal vesicle 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 formative and food yolk. The latter is of 

 a more oily nature than the former, and is usually not 

 segmented with the egg. The structure of the hen's 

 egg is more complicated. The outside shell consists of 

 earthy matter (lime) deposited in a network of animal 

 matter. It is minutely porous, to allow the passage to 

 and fro of vapor and air. Lining the shell is a double 

 membrane (membrana putaminis) resembling delicate 



