membrane of the yolk, as a result of which there are 

 independent yolk ducts for each embryo. The latter lie so 

 close to each other that a third one cannot be situated 

 between them; their head ends are especially close. 



Later, Wolff described the folds on the surface of the 

 yolk between the embryo and the openings of their intestines 

 into the yolk. In the structure of the embryos themselves 

 he did not notice anything deviating from the normal. 



In a special section of the work (Corollaria quaedam) 

 Wolff turned particular attention to the folds of the yolk 

 membranes, which are located between the embryos, He 

 believed these to be the remnants of the false amnion, which 

 from the first to the fifth day was present there. Therefore 

 his opinion was that the false amnion and area pellucida were 

 common for both embryos at the preceding stages. Comparing 

 the described case with examples of joined twins in the 

 literature, Wolff expressed his belief that the growing 

 twins do not result from the growth of the already formed 

 embryos, admitting at the same time that they are a product 

 of incomplete division of the embryo at the early stage. 



In the work Wolff reached no general conclusions. 

 However the distinction of the egg with two yolks, where the 

 presence of two embryos is entirely natural, from the twins 

 originating from one yolk is very obvious. Wolff undoubtedly 

 wanted to demonstrate a phenomenon which supported the theory 

 of epigenesis. The presence of two embryos connected with 

 one yolk indicates that the preformation of a ready embryo in 

 the egg does not exist and that the development occurs of 

 epigenesis . 



Wolff's second teratological work again is concerned 

 with the disturbance in the development of the chicken 

 embryo. 2 This is a small note with the following content. 

 In the spring of 1777, some of the eggs under a sitting hen 

 did not hatch chickens. On opening such eggs, it was revealed 



2. Wolff, "De pullo monstroso, quatuor pedibus, totidemque 

 alis instructo, ACTA ACAD. SCIENT. PETROPOLITANAE , 

 1783/ PP. 203 - 207. 



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