The Development of the Heart in Shad. 261 



There seems to be nothing unusual in the method of development 

 of the heart valves. The sinu-a trial valves (two, right and left) 

 are developed very late. I have serial sections of an embryo of 

 114 hours (No. 14,994) in which the heart has apparently ceased 

 to beat during atrial systole. In this specimen the (valveless) venous 

 orifice of the atrium is tightly contracted as by a sphincter. There 

 is little doubt that this specimen indicates the normal mechanism 

 of atrial systole prior to the formation of the sinu-atrial valves. 



The true circulation does not begin with the initiation of rhyth- 

 mical contraction in the, partially formed, heart-tube (36 somites), 

 but with the completion of the aorta (42 somites), which occurs at 

 a time when the heart-tube has been completely formed. Prior to 

 the division of the primitive heart-tube into conus ventricle and 

 atrium, the mechanism of systole of the entire tube is, in all proba- 

 bility, similar to that described for the atrium. 



Syracuse, N. Y., November 2, 1908. 



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