Chapter XIII 



THE PREFUNCTIONAL AS CONTRASTED WITH 

 THE FUNCTIONAL PERIOD OF DEVELOPMENT 



§1 



It has aheady been noted that some, at least, of the field-organi- 

 sation, both total and partial, characterising the early stages of 

 development, appears to persist throughout life, side by side with 

 the organisation characteristic of later stages. However, the de- 

 velopmental consequences of the new processes initiated in the 

 functional period are very striking and overshadow most of the 

 effects dependent upon field-organisation. 



These new processes fall under several main heads — growth, 

 true functional modification, the unification of the organism by the 

 nerves, and endocrine influences. It is impossible within the scope 

 of this book to give any detailed treatment of development during 

 this functional period, but a few instances may be presented which 

 will serve to make its main characteristics clear. 



The true growth-period of the embryo or larva does not begin 

 until the organism can either feed for itself, draw upon a store of 

 accumulated food material (as in meroblastic eggs), or be nourished 

 by its parent. Previous growth takes place only by imbibition of 

 water, or by slow contact absorption of yolk. Without quibbling 

 over precise definitions of growth, however, it may be pointed out 

 that the determination of organs may take place without any process 

 of growth being involved, and that growth may and normally does 

 continue long after tissue- differentiation has occurred. 



It appears, however, at least in some cases, as in that of axolotl 

 limb-buds, that degree of differentiation is correlated with absolute 

 size of the rudiment. If the rudiment is experimentally enlarged, 

 as by grafting one limb-bud on to another, the resuking single 

 enlarged limb (see p. 223) shows accelerated differentiation as 

 compared with the normal limb of the unoperated side^ (fig. 201). 



^ Filatow, 1932. See also Guyenot and Schott^, 1923. 



