the fold of the stomach, he noted that the "membrane con- 

 stituting the substance of the stomach does not break down 



at the border of the fossa , and leads into the internal 



layer of the transparent part of the vascular area and then 

 into the internal layer of the yolk itself." 



There is no doubt that this observation should have been 

 discussed as showing the continuity of the layers of the 

 embryonic and extra- embryonic entoderm which is connected with 

 the visceral layer of the mesoderm. Wolff saw the embryonic 

 layers, but he did not understand their significance. The 

 theory of the embryonic layers represents the product of 

 the first half of the nineteenth century and is connected 

 with the names of the Russian acadamicians Pander and Baer. 

 Wolff only established the factual basis on which this theory 

 could be later constructed. 



10. DE FORMATIONE INTESTINORUM, p. 454 



104 



