STRUCTURE AND DEVELOPMENTAL RATE 219 



It is further evident that 'identical twins' and the components 

 in double individuals need not necessarily exactly resemble each 

 other as is commonly thought. The two members of the pair 

 may be structurally very different ; in extreme cases one may be 

 normal and the other actually deformed. 



What influences could act on the smaller component that do 

 not also act on the larger? Evidently there can be nothing in 

 the external environment that would not come in equal contact 

 with both components, since they are intimately united and 

 enclosed within the same egg membrane. There can be no case 

 of injurious substances inducing a 'blastolysis' in the one com- 

 ponent and not acting on the other, or causing an early 'cellular 

 disorganization' (Kellicott, '17), which would not affect both com- 

 ponents. There is also no question of an insufficient supply of 

 nutriment of the ordinary type, since it is shown by many speci- 

 mens that two normal embryos equally as large as the usual single 

 one may develop on the yolk-sac of the fish's egg, compare the 

 first and last specimens shown by photograph in plates 1 and 2. 



There must, however, be some sort of competition between 

 the two components other than a competition for the appropria- 

 tion of ordinary yolk material. Much evidence suggests that 

 an interaction exists between the components similar to, if not 

 identical with, the interaction between two plant buds growing 

 from a common stock. When the growing tip is cut from the 

 shoots of certain plants, e.g., the ordinary privet, Lagustrum, as 

 a rule the axillary buds of the two leaves immediately below the 

 cut give rise to growing shoots. In many cases two shoots grow 

 at equal rates and are about equal in size, in other cases one of 

 the shoots evidently possesses some advantage and grows much 

 faster and becomes larger than its companion. Finally, in a few 

 cases a single vigorous shoot arises from one of the resting buds 

 and the opposite bud is entirely unable to grow. There is here 

 involved a factor in addition to available food material, just as 

 Loeb has found in the Bryophyllum leaf, and whatever this fac- 

 tor may be, through it the growing parts exert an inhibiting influ- 

 ence on one another. In the first case cited for the plant, the 

 growing impulse was balanced between the two upper axillary 



