THE PRIMORDIA 53 



passes through a series of values till a maximum is reached, just 

 as the length of a petal taken as example in § 45. When we 

 investigate the evolution of an individual ^ we may follow 

 the development of each primordium separately. We may 

 compare the evolution of several primordia in order to obtain 

 a view of the whole. We may also compare the develop- 

 ment of a given simple property in several individuals or 

 species. 



In order to make such comparisons easier I give here an 

 attempt to classify the primordia with respect to their develop- 

 ment. Considering first the primordia at the beginning of their 

 development, I bring them into three groups : 



(i) The original primordia, which appear (or exist) at the 

 beginning of the embryonic life of the individual (or of the 

 part of the individual to which they belong). Examples : In a 

 specimen of Spirogyra the total length and the number of cells 

 are original properties : they are both already discernible and 

 measurable at the beginning, when the specimen consists of 

 one cell (zygospore). ^ In a pluricellular simple hair of a plant 

 the total length and the number of cells are both original with 

 regard to the hair considered as a unit. The length and the 

 breadth of a petal are original primordia with reference to the 

 petal. 



(2) The metamorphic primordia, which are produced by a 

 transformation of a previously existing property. Examples: 

 A colouring substance, which appears at a certain moment in 

 a petal by a transformation of a chromogen. The presence of 

 hairs on a leaf (primordium hairy or downy) is a metamorphic 

 property, because each hair has its origin in a transformation 

 of an epidermal cell. In higher animals and plants the meta- 

 morphic properties are exceedingly numerous. 



(3) The awakening primordia, which are latent for a certain 

 time, and appear at a certain moment, independently of any 

 previously existing primordium. Awakening properties seem 

 to be rare. Example : The initial cell (zygospore) of a Spiro- 

 gyra is more or less oblong. After the second cell division two 

 of the four cells are cylindrical. The primordium cylindrical 

 appears at once : it is an awakening primordium. (See §§ 54 

 and 58.) It is sometimes difficult to determine whether a 

 given primordium is metamorphic or awakening. 



Considering the primordia at the end of their development 

 (independently of their origin), I divide them into two groups : 



^ Or of a part of an individual ; for instance, a petal, a leaf, an antenna of 

 an insect, a scale of a lizard, etc. 



* Here the value of the property total length is obtained by measuring the 

 length of the zygospore ; the value of the property number of cells is one. 

 The value of each property increases till the definitive (adult) value is reached. 



