ROUX: THE TWO PERIODS OF DEVELOPMENT 32 1 



periods cannot be sharply separated from one another, nor 

 does the transition from the one to the other occur at the 

 same time in the different tissues and organs. 



The conception is more fully expressed in 1905 as 

 follows : " This separation (of development into two periods) 

 is intended only as a first beginning. The first period 

 I called the embryonic period /car' eox>'iv, or the period 

 of organ-rudiments. It includes the 'directly inherited' 

 structures, i.e., the structures which are directly predetermined 

 in the structure of the germ-plasm, as, for instance, the first 

 differentiation of the germ, segmentation, the formation of 

 the germ-layers and the organ-rudiments, as well as the next 

 stage of ' further differentiation/ and of independent growth 

 and maintenance, that is, of growth and maintenance which 

 take place without the functioning of the organs. 



" This is accordingly the period of direct fashioning 

 through the activity of the formative mechanism implicit in 

 the germ-plasm, also the period of the self-conservation of 

 the formed parts without active functioning. 



" The second period is the period of ' functional form- 

 development.' It includes the further differentiation and 

 the maintenance in their typical form of the organs laid 

 down in the first period ; and this is brought about by the 

 exercise of the specific functions of the organs. This period 

 adds the finishing touches to the finer functional differentia- 

 tion of the organs, and so brings to pass the ' finer functional 

 harmony ' of all organs with the whole. The formative 

 activity displayed during this period depends upon the 

 circumstance that the functional stimulus, or rather the 

 exercise by the organs of their specific functions, is accom- 

 panied by a subsidiary formative activity, which acts partly 

 by producing new form and partly by maintaining that 

 which is already formed. . . . Between the two periods lies 

 presumably a transition period, an intermediary stage of 

 varying duration in the different organs, in which both 

 classes of causes are concerned in the further building-up of 

 the already formed, those of the first period in gradually 

 decreasing measure, those of the second in an increasing 

 degree" (pp. 94-6, 1905). 



In the first period the organ forms or determines the 



