TWINNING IN BIRDS 



83 



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two equivalent primordia (Fig. 40). The three meso- 

 blastic somites in the middle seem to belong in common 

 to the two embryos. The whole primitive streak region 

 seems to be single. In brief, the head-process region 

 back to the primitive knot has twinned and the post- 

 cephalic region has remained un twinned. Such dupli- 

 cities are technically called 

 anadidymi. 



As is the case in human 

 double monsters there is a 

 class of chick duplicities in 

 which the head region is more 

 or less completely single while 

 there are two widely diver- 

 ging trunks. A typical ex- 

 ample of this sort of mon- 

 ster, really a katadidymus, is 

 figured by Tannreuther 

 (Fig. 41, p. 84). The head 

 region back to the somites 

 is normal and single; pos- 

 terior to that there are two 

 bodies diverging at an angle 

 of about 120°. Tannreuther 

 considers that this '^double 

 embryo no doubt began its development as two inde- 

 pendent primitive streaks, with a .later connection or 

 fusion of the anterior ends of the two-head processes." 

 With this conclusion I cannot agree. It seems to 

 me inconceivable that two separate axes could come 

 together with such perfect symmetry as to form 

 a normal single head. In brief, the objections to the 



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Fig. 40. — A typical double- 

 headed chick monster (anadidy- 

 mus) resulting from the partial 

 dichotomy of the anterior end of 

 the originally single embryonic 

 axis. (After Tannreuther.) 



