white spot, which later is converted into the top of the beak. 

 Simultaneously with the conversion of the beak into a 

 cartilaginous mass, an elevated white spot appears on it 

 (57) , having a top which on the tenth day is the size of a 

 small pinhead. On the eighteenth day the top of this elevation 

 has the shape of a spike, which the chick uses at the time of 

 hatching to break the membranes and perforate the shell. 

 Within one to two days after hatching, the prominence on 

 the beak falls off; however, its remnants can be seen for 

 some time. 



INTESTINAL YOLK DUCT. This formation Tredern called 

 the stem or the apophysis. According to his data, the intestinal 

 yolk duct is formed on the fourth day of incubation, after the 

 closing of the intestinal canal, as a delicate extension as of 

 a narrow gut through the opening to the abdominal cavity (58) 

 (Figure 18, 19(5)). Later, the length of the duct increases, 

 and apparently it is composed of a membrane and a gut. The 

 connection between the yolk and the intestine through this 

 duct shows that it is possible to blow into the intestine 

 from within the yolk sac. Because this is not always possible, 

 some authors doubted the existence of an opening in the yolk 

 intestinal duct (59). However, other investigators, for 

 example Vicq d'Azyr and Blumenbach, regarded this experiment 

 as successful. The remains of the yolk intestinal duct are 

 seen after hatching in the form of a narrow mesentery 

 (Figure 18, 27(2)). The remains of the mesentery can be 

 observed up to the seventeenth day after hatching, and in 

 some birds, especially swimming birds, throughout its life 

 (Figure 18, 31(2)). 



INTESTINAL CANAL. Tredern considered the digestive 

 canal only from the stomach to the anus, and he described 

 its changes during incubation. On the fourth day it is 

 possible to see the intestinal canal with a lens (60) . In 

 this period it has an equal diameter throughout its extension. 

 Tredern noted the great accuracy of Wolff's work in the NEW 

 COMMENTARY OF THE PETERSBURG ACADEMY OF SCIENCE, 

 especially in his third section. Because Wolff had studied 

 intestinal development under a microscope, and not with a 

 lens as Tredern did, the latter did not consider that he 

 could add anything to Wolff's description. 



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