THE PRIMORDIA 61 



§ 49.— THE QUANTITATIVE METHOD APPLIED ON 

 EMBRYOLOGY. CURVES OF DEVELOPMENT. LEAD- 

 ING PROPERTIES. — In § 47 I have given a short account of 

 the development of certain properties of plants. The examples 

 given are very simple and the facts mentioned may be observed 

 without comphcated technique, but the principles of the method 

 are applicable to the study of any embryological subject. 

 These principles may be summarized in the following way : — 



(i) In the study of the development of a Uving individual (or 

 part, segment, organ, etc.) we want to discover a certain number 

 of simple properties (primordia). 



(2) Since the primordia are on the whole independent of 

 each other, the development of each primordium ought to 

 be studied separately, by measuring its value at successive 

 moments of the period of development. 



(3) The primordia of a species are very numerous, in higher 

 animals and plants practically unlimited in number. There- 

 fore it is advisable to concentrate our attention upon the 

 development of a small nvraiber of well-chosen properties. 



(4) The successive values of each property ought to be set out 

 in order in a chronological series. They may be represented by 

 ordinates, by means of which a curve of development may be 

 drawn. 



(5) The moment at which a transitory property is trans- 

 formed into a metamorphic one ought to be determined and 

 indicated in the curve of development. In a similar way 

 the moment at which a (caducous) property disappears or a 

 (awakening) property appears ought to be determined and 

 indicated in the curve. 



(6) The curves (or series of figures) of all the investigated 

 properties ought to be brought together into one table or one 

 diagram (§ 47, first example). By means of an embryological 

 table, or diagram constructed in that way, we may compare 

 exactly the lines of development of several properties ; we may 

 find which is, at any given moment, the existing combination 

 of values ^ ; we may obtain a clear notion of the way in which 

 a given state proceeds from the preceding state and gives birth 

 to the next one ; we may obtain valuable information about 

 the segregation of the properties, for instance, with reference 

 to the developmental continuity or discontinuity between two 

 properties, and we may compare several individuals, species, 

 etc., by comparing their tables or curves of development. 



(The gradation curves mentioned in Part VIII. are constructed 

 on the whole according to the above principles. See Fig. 27.) 

 In the third and fourth examples described in § 47 I have 



1 In other words (using the ordinary language), we may find which are the 

 characteristics of a given embryo at any moment of its development. 



