In order to substantiate his ideas, Shchurovskii referred 

 to the observations of "Malpighi, Haller, Wolff, Spallanzani, 

 Pander, Preyost, and Dumas." He distinguished six stages 

 or steps of development in the heart of the chick. The first 

 (about twenty-seven hours after fertilization) produces an 

 elongated straight tube, both, ends of which disappear in a 

 transparent fluid. In the second stage (about thirty-one 

 hours), the heart already has two venous and three to four 

 arterial branches, and there is marked movement of colorless 

 blood. In the third stage (thirty-six hours), the heart is 

 characterized by the curvature of the heart tubing and two 

 interceptors, "one of them separating the first cavity from 

 the second, which is called the atrial system, and the other 

 going from the second cavity along the span of the descending 

 trunk, which is named the ventricular system." The first cavity 

 he called the auricle; the second, the left ventricle; and 

 the originating stem, the aorta, which by the end of the thirty- 

 ninth hour is divided into two stems, each of which develops 

 side branches . In the fourth stage (forty-eight hours) the 

 heart changes from its elongated form to an oval or spherical 

 form. At this time the heart appears "with one auricle and one 

 ventricle," and the colorless fluid is replaced by red blood. 

 In the fifth stage (fifty-eight hours) the right ventricle is 

 formed through the extension of the vessel which runs from 

 the right part of the auricle to the ventricle. In this period 

 the heart has one auricle and two ventricles. Finally, at the 

 sixth stage (third day) , the cavity of the auricle is divided 

 into two and forms a two-auricle, two-ventricle heart. 



The essential error of this description is in the statement 

 that the ventricle is divided into two before the auricle. 

 This mistaken assumption of the presence of a one-auricle, 

 two-ventricle heart, as will be seen later, caused Shchurovskii 

 serious difficulty. 



For a description of the development of the avian heart, 

 we can compare its embryonic stage with the development of 

 the heart of different animals. This is based on evidence 

 that "the different periods of formation of this organ have 

 a separate, independent life, and in their combination 

 represent one organ, whose gradual development takes place 

 in separate special organs in the different classes of 

 animals" (pp. 197 - 198). 



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