REGENERATION 171 



the other cells within the field to form subordinate organs in 

 accord with their place in the field. 



If, therefore, a tissue is to form a new apical region, it is 

 necessary for it to be removed from the inhibitory influence 

 of the existing apex. This can be done by cutting the latter 

 away, but in normal development it may occur as a consequence 

 of growth. The field emanating from the "dominant" apex has 

 a limited extent, and consequently part of the tissue can be 

 removed from its sphere of influence by growth — so-called 

 ''physiological isolation" (Child). This tissue will then form a 

 new apical region. This is the law governing the occurrence 

 of new buds in colony-building organisms, such as hydroid 

 polyps. We have seen that the extent of the field depends upon 

 the state of phj^siological activity of the tissues. The distance 

 between buds formed under favourable conditions of food, 

 temperature, etc., will therefore be greater than that between 

 buds formed under less favourable conditions. This may 

 completely modify the appearance of the colony as a whole 

 (Child, 1929). A similar phenomenon occurs in some planarians 

 which reproduce asexually by transverse division. Here, at a 

 certain distance from the original head of the individual, the 

 caudal part forms a new head. If, under unfavourable circum- 

 stances, the size of the old head is reduced (see p. 168), the 

 field emanating from it extends less far, and division takes 

 place at a much smaller size of the body. 



Child has demonstrated in many experiments that the 

 dominant apical parts are often highly sensitive to noxious 

 influences, and that this sensitivity decreases in a caudal (or 

 basal, respectively) direction. He ascribed this to differences 

 in the "physiological (i.e. metabolic) activity" of the tissues, 

 which would be highest in the apical parts, and decrease 

 gradually from there. Similar results were obtained with an- 

 other method for the demonstration of differences in the in- 

 tensity of metabolism, viz. measurements of the velocity of 

 decolorisation of certain redox dyes, such as methylene blue, 

 at low oxygen pressures. From these differences in physio- 

 logical activity. Child explained such morphogenetic influences 

 as "dominance" and "organisation", exerted by the apical part. 



