198 CHARLES R. STOCKARD 



6. STRUCTURAL DIFFERENCES BETWEEN THE TWO COMPONENTS 

 IN CONNECTED TWINS AND DOUBLE INDIVIDUALS 



As illustrated in plates 1 and 2, the components in connected 

 twins and double individuals exhibit various degrees of separate- 

 ness from partial double-headedness to completely double indi- 

 viduals. It has also been brought out in the previous section 

 that the degree of doubleness shown by any such specimen de- 

 pends upon the original distance apart of the two embryonic 

 shields along the germ-ring of the fish's egg, as illustrated in the 

 diagrams of figure 11. As Morrill ('19) has pointed out, the dif- 

 ferent extents of doubleness are in no way connected with differ- 

 ent times of origin of the condition as was suggested by Newman 

 ('17, p. 17-18), since every extent of doubleness is shown in this 

 fish series and the time of origin from the developmental stand- 

 point is the same in each case. 



Irrespective of the degree of doubleness or the distance apart 

 of the two components, there is a most significant competition, 

 so to speak, between the components themselves, just as exists 

 among several buds growing from a common stock. It is the 

 results of this interaction or competition between the two com- 

 ponents which we wish to consider in the present section, and 

 their bearings, of very general importance, will be analyzed in the 

 sections following. 



Of. Double individuals with identical or equal-size components 



The two components in each of the specimens photographed 

 in plates 1 and 2 are practically of equal size. The first plate 

 illustrates the young trout from a dorsal view and the second 

 plate shows the same individuals arranged in the same order from 

 the ventral aspect. On comparing the two views of every speci- 

 men, it will be found that all heads are perfectly normal in appear- 

 ance, each having two fully developed eyes, a perfectly formed 

 mouth and branchial structures and a perfectly developed bilat- 

 eral brain with its general contour clearly visible below the skin. 

 On further comparing the two views in a given specimen, the body 

 regions of the components are also found to be about equally 



