1897] FUNDAMENTAL PRINCIPLES OF HEREDITY 235 
The most familiar is the simple halving of the cell each time it has 
attained double its original bulk (Herbert Spencer's ‘limit of growth’), 
a process termed in Hibernian phrase ‘multiplication by simple 
division. Sometimes, however, the first division is followed im- 
mediately by another, and so on, so as to produce with little delay 
grandchildren or great-grandchildren, &c.; this process is called 
‘brood-division, or, when the progeny do not immediately separate, 
‘segmentation. Again the progeny of brood-divisions may as- 
semble in groups, usually in pairs, which fuse to form a new or 
‘coupled-cell’; this process is called ‘conjugation, or, if the 
‘pairing-cells’ are dissimilar, ‘fertilisation. We must bear 
in mind that conjugation processes are not, strictly speaking, 
processes of multiplication; for the act of pairing halves the total 
number of cells for the time being, one replacing two: the two 
literally become one flesh. 
We very often find these three reproductive processes recurring in 
cycles, ¢.g., a succession of simple divisions at the limit of growth is 
wound up by brood-formation, and the brood-cells conjugate ; the 
coupled-cell then initiates a fresh cycle. But the order of the processes 
varies in different cases, and sometimes even different modes of brood 
division may alternate. Thus a common Gregarine, parasitic in the 
Earthworm, shows the following: after conjugation the coupled-cell 
undergoes repeated brood divisions so as to form many hundred of 
brood cells; each of these matures into an oat-shaped body sur- 
rounded by a hard shell. After a time the oat-shaped cell divides 
again by brood formation into eight sickle-shaped cells, which finally 
leave the oat-shaped case and migrate into the living cells of the worm. 
In many cases the separation of the daughter- or brood-cells is 
not complete, and they remain associated in more or less close union. 
Such an assemblage of cells of common origin is called a biological 
‘colony’ in the strict sense, the term ‘social aggregate’ 
being used for an assemblage formed like a human colony by the 
flocking together of originally isolated organisms. Protist colonies 
may he ee in three ways, the third being only a combination of 
the first two : 
(1) Cell division, alternating with intervals of aes gives 
rise to daughter-cells which remain united together. 
(2) Brood division (segmentation) produces a number of 
cells which remain united together. 
(3) A colony first formed by segmentation continues to enlarge 
by the division after growth of its several cells, the 
daughter-cells still remaining connected. 
Colonies of the first and third type may be propagated by the 
separation of a part of the colony ; if the separated part consist of a 
single cell this merges into true reproduction. 
