that eggs bearing twins were known by Harvey and Fabricius 

 Aquapendente and even Aristotle. However, all these authors 

 worked not with single eggs which contain one simple yolk 

 with two embryos developing in it, but with eggs containing two 

 yolks, each of which gives rise to a separate embryo. Wolff 

 wrote that he also saw many eggs with double yolks; he 

 suggested naming them the twin-eggs in distinction from the 

 single eggs carrying twins (p. 457). He considered the methods 

 of development of this and other types of eggs. For the 

 formation of twin eggs it is necessary that two yolks be 

 separated in the ovary and be passed separately along the 

 oviduct, after which in the lower part of the oviduct they 

 are surrounded by a common white mass and covered with a 

 common shell. Two yolks in one egg could be closely adjacent 

 to each other; however, they remain separated and represent 

 at the end two different eggs in one shell. Something else 

 completely is represented by the egg carrying twins, which 

 contains a single and is actually single in every sense. 

 Such an egg is single throughout its entire existence but 

 nevertheless produces two embryos. 



The egg described in this work, which carried twins, 

 was incubated for six days. Wolff gave a short description 

 of the situation of the parts in a normal egg. At this time 

 of incubation the albumin is separated from the yolk and 

 is located at the pointed end of the egg. The vascular zone 

 (area vasculosa) occupies half of the surface of the yolk 

 and is supplied with rare vascular branches. The transparent 

 zone (area pellucida) , which represents the central part of 

 the vascular zone, i.e. the place where the embryo is 

 located, disappears by the sixth day of incubation in normal 

 eggs. 



In the egg carrying twins, the albumin is single and 

 common for both embryos and normal in size and location. 

 The yolk also is single and does not show anything abnormal 

 (p. 465). The vascular zone is also single; however, the 

 vessels present in it form a double system of branching which 

 is considered the first sign of doubling of the embryo. 

 The strange property of this egg is the absence of the 

 amnion; thus both embryos look entirely unusual: they are free, 

 covered with nothing, and lie movable on the yolk (p. 469). 

 Skin of the abdomen in both embryos gets across in the 



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