INDIVIDUATION AND REPRODUCTION 223 



other conditions which decrease the rate of metabolism, the head is 

 smaller and develops more slowly, the pharynx appears much nearer 

 the head, and the new prepharyngeal region is correspondingly 

 shorter (Figs. 97, 98). In extreme cases the head may be terato- 

 morphic (Fig. 99), or even anophthahnic (see pp. 111-12), and 

 no reconstitution occurs posterior to it. In similar pieces, under 

 conditions which increase the rate of metabolism, such as high 

 temperature, the prepharyngeal region is longer and the pharynx 

 appears farther from the head (Fig. 100). Evidently the distance 

 from the anterior end at which certain conditions arise in the piece 

 under its influence varies with the rate of metaboHsm in the domi- 

 nant anterior region. When the rate is very low the anterior region 

 does not bring about any visible change in regions posterior to itself, 

 and the higher the rate the greater the distance at which particular 

 changes occur. 



In the higher animals, such as the vertebrates, as well as in the 

 higher invertebrates, the size of the adult individual is limited by 

 other factors than the hmit of dominance, so that such animals 

 never attain anything like what might be called the physiological 

 maximum of size. The chief Hmiting factor in these cases is 

 apparently the higher degree of dififerentiation of the cells which 

 results in the retardation and sooner or later in the almost complete 

 or complete cessation of growth. Only in those forms in which 

 agamic reproduction occurs can we be certain that the individual 

 attains the physiological maximum, i.e., the size determined by the 

 limit of dominance. In the adult stages of the higher animals 

 dominance may extend to almost indefinite distances, but individual 

 size is limited by differentiation and lack of capacity for indefinite 

 or long-continued growth. Even in these forms, however, the size 

 of parts and their repetitive reproduction during development may 

 be determined by the Hmits of dominance in the early stages. 



When we consider all these facts and many others, some of which 

 have been mentioned elsewhere' but cannot be discussed here, we 

 are forced to conclude that a relation of dominance and subordi- 

 nation of parts in the organism really exists, that it is effective 

 only within a certain spatial hmit, varying with conditions in the 



' Child, 'iia, '116, 'lie, '13a, 'lib, 'nc. 



