THE FUNCTIONAL PERIOD OF DEVELOPMENT 437 



negligible or absent in another. For instance, it is impossible for 

 holometabolous insects to produce functional modifications 

 during individual ontogeny in their skeletons. The hard parts of 

 these animals are definitively formed, with all their adaptive de- 

 tails, on emergence from the pupa, and no further growth is 

 possible. The same is true for the development of their muscles 

 and tendons : these must be preformed during the pupa stage so as 

 to permit of perfect function and locomotion of the animal as soon 

 as they are called upon. 



There is thus a remarkable contrast between the development of 

 vertebrates and that of higher insects. In the former, prefunctional 

 differentiation lays down a rough sketch of the organism, upon 

 which most of the finer adaptive details are later inserted by means 

 of functional response to the demands made upon the parts. In the 

 latter group, on the other hand, although doubtless some details, 

 such as those of the blood-vessels, may be determined through 

 functional response, the greater part of the structure, including 

 even the finer adaptive details, must be laid down by elaborate 

 chemo-diflFerentiation, unaided by functional response. 



There are, of course, other equally fundamental diflFerences in 

 developmental methods between groups. Hormones play a very 

 large part in the later stages of vertebrate morphogenesis ; but in 

 insects their role appears to be altogether subsidiary. Similarly, 

 the adult form of a vertebrate is determined by changes in pro- 

 portion of parts which are brought about by diflFerential growth in 

 already functioning organs, and which continue through a large 

 fraction of the Hfe-span; in holometabolous insects, no growth 

 occurs in differentiated parts, and proportions must be definitively 

 fixed during the short pupal period. 



The subordination in Ascidians of the period in which the total 

 gradient-field system is the sole form of organisation, as contrasted 

 with its long persistence in Amphibia, is another example, in this 

 case concerning early stages of development, of the diflFerences 

 which may exist between groups as regards their developmental 

 mechanisms. 



