ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 75 



roof, or that other factors exist capable of controlling asymmetry, 

 but normally overruled by the gut-roof factor. 



i\nother line of attack on this problem is provided by those ex- 

 periments in which a blastula of a newt is partially constricted by 

 tying a fine hair round it in the plane of bilateral symmetry. The 

 result is the production of double-headed monsters, and, while the 



Fig. 32 



Anterior doubling producing situs inversus viscerum et cordis in the right-hand 

 member. The doubhng was produced by partial constriction in the plane of 

 symmetry of an early cleavage stage of Triton. The heart, gut, and position of 

 liver (L.) and pancreas (P.) of the right-hand member (seen on the left in this 

 ventral view) are reversed. (After Spemann and Falkenberg, Arch. Entwmech. 

 XLV, 1919, simplified.) 



left-hand member of such a pair always shows the normal asym- 

 metry, the right-hand member nearly always shows situs inversus } 

 Double-headed monsters also occur in trout, in wild conditions and 

 in hatcheries. When the two members are joined together only by 

 the hinder region of the trunk (behind the abdominal cavity), both 

 members have the usual vertebrate asymmetry. But when the join 

 between the two members is farther forward, so that the alimentary 

 1 Spemann and Falkenberg, 19 19. 



