CONJUGATION. 39 



parent. Buds are usually formed on the outside of the 

 parent but occasionally internal buds occur. In many cases 

 the buds may remain in organic contact with the parent, 

 when a compound organism or colony is produced. 



Binary fission is the usual method of cell-reproduction 

 throughout the animal kingdom. - In unicellular organisms, 

 such as Amaba, the nucleus divides into two equal parts 

 with complex changes, called mitosis (see Chapter IV.), and 

 the cell follows suit. Each fresh cell then grows and, when 

 each nucleus and cell has reached the limit of growth, a 

 fresh binary fission takes place. 



We may illustrate this process by a diagram (Fig. 13). 



In this manner growth and reproduction alternate, and 

 the relationship of cell to nucleus, and of surface to bulk, is 

 maintained at the normal. 



In a multicellular individual the constituent cells grow 

 and multiply in the same manner, and the same diagram 

 will serve if we recollect that the cells are aggregated into 

 one compound individual instead of becoming separate 

 organisms. 



Returning to the unicellular organism, we might perhaps 

 suppose this cycle of alternate growth and reproduction 

 by binary fission to be capable of infinite repetition, but 

 such is not the case. After a certain number of repeti- 

 tions another process intervenes called Conjugation. 



Conjugation consists of two series of events— (i) Pre- 

 paratory reduction of nuclei in two individuals, and (2) 

 Interchange of nuclear substance : — 



1. Preparatory Reduction of the Nuclei. — Two indi- 

 viduals join together in such a way that their proto- 

 plasm is continuous. All activity is suspended and the 

 nucleus of each increases in bulk. Each nucleus then 

 divides into two and into four by binary fission. 



2. Interchange of Nuclear Substance. — Each individual 

 now has four portions of the nucleus in its substance. 

 Two of these are absorbed and disappear, whilst one 

 from each individual moves across into the other individual, 

 and each of these migrants then fuses with the part 

 still left to form a compound nucleus. The individuals 



