66 THE PHYSIOLOGY OF TWINNING 



for the interruption of development. The abnormal 

 component is viewed as the product of a secondary or 

 lateral bud. An analogy is presented between this 

 condition and that seen in certain plants with a terminal 

 growing-point. So long as the terminal growing-point 

 (equivalent to the primary bud or the normal embryo) 

 retains its normal rate of growth, secondary buds are 

 inhibited; but if the prin^ary bud be injured or removed, 

 secondary buds (equivalent to the smaller embryo) 

 arise and grow, but are often partially inhibited by the 

 presence of the primary bud. This theory seems vaguely 

 to imply that the smaller component arises in some way 

 from the side of the primary axis like a lateral branch, 

 while the original head remains intact as the head of 

 the larger component. Possibly, however, no such crude 

 analogy is intended, but we are merely meant to infer 

 that a smaller ''secondary bud" or embryonic shield 

 arises on the germ ring and that through the process 

 of concrescence the primary and secondary individuals 

 fuse in such a fashion as to give us individuals duplex 

 anteriorly and simplex posteriorly. Whichever of these 

 alternatives is meant, one is as untenable as the other. 

 The lateral budding idea is quite incompatible with the 

 fact that even when two components are distinctly 

 unequal they contribute quite symmetrically of all 

 their median organs to the single part of the body. 

 Such an idea would involve the assumption that budding 

 began internally so as to involve notochord, neural tube, 

 and all other median structures, and that these struc- 

 tures divided equally between the stock and the bud — 

 a process that would be not budding at all but fission. 

 The second alternative involves the old-fashioned view 



