METHODS FOR OBSERVING THE DEVELOPMENT 



421 



IXX 



Formation of fundamental regions of chick heart by progressive fusion of its paired primordia. A. At 9- 

 somite stage, when the first contractions appear. The ventricular pKirt of the heart is the only region 

 where the fusion of the paired primordia has occurred and the myocardial investment has been formed. 

 B. At 16-somite stage, when the blood first begins to circulate. The atrium and ventricle have been 

 established, but the sinus venosus exists only as undifferentiated primordial channels, still paired, and 

 still lacking myocardial investment. C. At 19-somite stage. Fusion of the paired primordia is just 

 beginning to involve the sinus region. D. At 26-somite stage. The sinus venosus is definitely estab- 

 lished and its investment with myocardium well advanced. To facilitate understanding of the progress 

 of fusion, arabic numerals have been placed against approximately corresponding locations. The 6 is 

 located at the point of entrance of the duct of Cuvier as determined from injected specimens. (From 

 Hoff, Kramer, DuBois, and Patten. Am. Heart]., 1939(2).) (Reproduced by courtesy Dr. B. M. Patten 

 from Am. Naturalist 39:225, 1951) 



