CLASSIFICATION OF THE PRIMORDIA 71 



each other and are divided in their turn into 2,4 . . . 2« cells. ^ 

 All the successive divisions take place according to the already 

 mentioned axis of segmentation and all the cells remain adher- 

 ent. In this way an adult specimen of Spirogyra is produced. 

 This is a uniaxial system of segments (Fig. 2). 



I call this object a specimen. It may be called an individual. 

 Unfortunately the latter term is used in different senses ; it 

 may be applied to the egg, to the adult pluricellular specimen 

 and to each of its cells. ^ The pluricellular specimen may be 

 looked upon as being an individual of a certain order x. Tak- 

 ing this individual as starting-point, I call each of its unicellular 

 segments (which are all equivalent) an individual of the order 

 x-\-i.. In the present example we may put down x = i.\ accord- 

 ing to this, the pluricellular specimen is an individual of the 

 first order and each cell an individual of the second order. ^ 

 I don't go further, looking (in the present work) upon each cell 

 as being an individual (unit) of the last (simplest) order. 



Generally speaking, I call individual of the order x or indi- 

 vidual X any specimen, individual, segment, organ or part 

 whatsoever * of which I want to investigate or to describe the 

 constitution. An individual x may be divided by segmenta- 

 tion into individuals x + i\ these may consist in tiieir turn of 

 individuals ^c + 2, etc. 



REMARK : The above definitions are based upon the 

 principle that the notion of individual is a relative one. The 

 definition of the term individual involves a convention. In 

 biology we use this term in the sense of unit ; the definition of 

 any unit is a matter of convention. Examples : the metre is 

 the unit of length in the metric system. Since we know that 

 an error has been committed in the original measurement of 

 the metre, this unit is conventional. Moreover, we have been 

 compelled to adopt a series of other units of length ; for in- 

 stance, the micron, the millimetre, the centimetre [C.G.S. 

 system), the kilometre, etc., the standard being the metre. If 

 the founders of the metric system had been pure logicians they 

 might have taken the metric ton (1000 kilogrammes) as the 



1 Certain particularities, which are of no importance for our subject, are 

 passed over in silence in this description. See about the first cell-divisions in 

 Zygnema : N. WILLE, in ENGLER and PRANTL. Natiirl. Pflanzenfam., 

 Theil I, Abt. 2, Fig. 11. See about Spirogyra: G. S. WEST, British Fresh- 

 water Alga (Cambridge, 1904), Fig. 49. 



^ It happens under certain conditions that the cells separate from each 

 other and become really independent individuals. 



^ It might be more logical to call each cell an individual of the first order, 

 and the pluricellular specimen an individual of the second order, but in com- 

 plicated cases it is very difficult or even impossible to adopt this notation. 

 See note i, p. 72. 



* A tree, a moss, a leaf, a hair, a Spirogyra, an insect, an antenna, etc., 

 and by further extension a shell of a mollusc, the shell of an egg of a bird, etc. 



