In 1827 Baer published an article on a double chicken 

 embryo at the beginning of the third day of incubation. 1 

 The purpose of this work was to solve the disputed question 

 of the origin of double monsters. "I never could bring 

 myself to accept the idea," Baer wrote, "that double monsters 

 arise from the union of two individuals, although previously 

 many physiologists insisted on this opinion. It seems to me 

 that here one difficulty has replaced another" (p. 576) . 

 The arguments proposed by Baer against the theory of union 

 were very close to those used by Wolff in his "Description 

 of Two-Headed Calf, accompanied by general illustrations on 

 the origin of monsters." 2 In order for two embryos to unite 

 in an earlier stage of development, it would be necessary 

 for them to be completely movable and for there to be 

 pressure upon them from two sides, pressing them together. 

 But the main difficulty lay in the fact that it is impossible 

 to imagine how the corresponding parts of the two individuals 

 meet and why the union takes place without disturbing the 

 topography of the parts. More naturally, Baer supposed 

 this case to represent the splitting of a formerly unified 

 rudiment. This explanation did not eliminate all the diffi- 

 culties; it was suitable for the interpretation of lateral 

 splitting, but left unexplained the cases of divergence of 

 surfaces and union of twins by the ventral sides. 



Baer thought that the solution to this interesting 

 question could be achieved only through the study of the 

 early stages of development of double monsters. He described 

 one of these cases, in a developing chick at the beginning of 



1. Baer, "Uber einen Doppel-Embryo vom Huhne aus dem Anfange 

 des dritten Tages der Bebrutung," ARCH. ANAT . PHYSIOL. 

 (1827) , pp. 576-586. 



2. K. F. Wolff, "Descriptio vituli bicipitis . . . ," NOVI 

 COMMENT. ACAD. SCIENT., PETRPOL.,32 (1773), 



pp. 504-573 (see Chapter 5) . Baer did not refer to this 

 work by Wolff. Baer's exceptional honesty, which appears 

 in all his works, suggests that Wolff's paper on the 

 double-headed calf was for some reason unknown to Baer. 



454 



