170 



PRINCIPLES OF ANIMAL BIOLOGY 



divide, a half going to each new individual. After the separation into two 

 individuals, regeneration is completed and each individual grows in size. 

 As stated on page 164, fission occurs every 16 to 24 hours in a healthy line 

 of paramecia. 



In the reproduction of certain parasitic protozoa the nucleus of a 

 large cell may divide many times without the division of the cytosome. 

 Later the cytosome divides, not by successive equal fissions but by 

 many simultaneous divisions, into as many pieces as there are nuclei, 

 thus forming a number of small cells at the same moment. This process 

 is sometimes called multiple fission and sometimes sporulation. It 

 occurs regularly in the complicated life history of the organism of malaria. 



Budding. — When an organism divides unequally, the reproduction is 

 termed budding. The larger portion may be regarded as the parent, 

 the smaller one as the offspring. Usually, also, there is a definite protru- 



■^^mfM,^ 



Fig. 139. — Gernmule of fresh-water sponge (left), and young sponge recently emerged from 

 gemmule: os, osculuni; sp, spicule. {Gemmule after Hesse and Doflein.) 



sion of the bud, which is small at first but grows larger. The bud usually 

 develops organs similar to those of the parent and either becomes inde- 

 pendent of, or remains attached to, the parent. Budding is a rare 

 reproductive process among the protozoa but is common in certain 

 groups of the metazoa. 



In the metazoa the budding may be either internal or external. In 

 the former, the buds are formed somewhere within the body substance ; 

 in the latter, they are on the surface. 



Internal Budding. — In fresh-water sponges, masses of cells collect 

 in the jellylike middle layer of the body wall. Hundreds of cells are in 

 each mass, and around them is a horny layer which often contains many 

 spicules. Such a reproductive body is called a gemmule (Fig. 139, left). 

 The gemmules are not shed, but when the parent's body disintegrates at 

 the end of the season, they are left exposed on the log or stone to which the 

 sponge was attached. They may remain there, or they may be trans- 

 ported considerable distances by water currents or perhaps by the feet 

 or beaks of birds. 



With the return of favorable (conditions the bud enclosed within the 

 outer coating. of the gemmule begins to develop. There is an opening at 



