46 THE PHYSIOLOGY OF TWINNING 



to invaginate and to form an axis, may become inhibited 

 through the much more active growth of the primary- 

 axis and may become obliterated. Stockard, for example, 

 noted a number of cases in which secondary embryonic 

 shields appeared but no accessory embryos were formed. 

 If, however, the secondary embryonic shield is allowed 

 to develop far enough it cannot be completely suppressed 

 by the primary embryo and a secondary or subordinate 

 embryo will develop. Such an embryo will always, I 

 believe, exhibit some evidences of being inhibited, such 

 as small size, cyclopia, and other defects. The primary 

 embryo, as in the starfish larvae, may grow quite 

 normally, apparently suffering no detriment from the 

 presence of the secondary embryo, until they have 

 mutually surrounded the yolk sac and thus come to lie 

 belly to belly, attached by means of the vitelline tissues. 

 When this happens the larger primary embryo tends to 

 grow around and absorb the smaller secondary embryo 

 and frequently succeeds almost completely in doing so. 

 The larger larva is here the autosite and the smaller 

 one the parasite (Figs. 25 and 26). It is my opinion 

 that the condition of autositism and parasitism in fishes 

 practically always arises in this fashion. Cases of false 

 autosites and parasites have been described by Stockard 

 in connection with true conjoined twins derived by the 

 separation of parts of a single embryonic axis. This 

 condition does not concern us here, for we are dealing 

 with twins derived by plural gastrulation. 



ORIGIN OF TRUE DUPLICATE TWINS 



Harking back once more to the starfish situation, 

 it may be recalled that a very common type of twin 



