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 Amada, 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—(1) Pre- 
paratory reduction of nuclei in two individuals, and (2) 
Interchange of nuclear substance :— 
1. Preparatory Reduction of the Nucleii—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 
