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 

 ' coup led- 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, e.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 be formed in three ways, the third being only a combination of 

 the first two : 



(1) Cell division, alternating with intervals of growth, 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. 



