DEVELOPMENT OF THE HUMAN HEART 



255 



does it project to the right side of the bulb which becomes its per- 

 manent position. This is seen in No. 463 (fig. 4). 



Although the two embryos just mentioned are practically of 

 the same stage of development, the difference of the degree of 

 development of their hearts is most pronounced. No. 463, which 

 is perfectly preserved, has a heart with larger atria, a more con- 

 stricted atrial canal, a large left ventricle and a pronounced but 

 contracted bulb. The muscular wall of the whole heart is con- 

 tinuous without a single break in it. That surrounding the atrial 

 canal is sharply defined forming a continuous ring connecting at 



RCu. 



Fig. 4 Section of the heart of an embryo 3.9 mm. long (No. 463). X 66. 



all points with the atria above and the left ventricle below. This 

 is mentioned especially because a share of this connecting ring 

 disappears while the remaining portion becomes the atrio-ventric- 

 ular bundle. 4 



B. FORMATION OF THE ATRIO-VENTRICULAR VALVES 



In the earliest stages described (No. 391), while the muscle 

 wall of the heart is still in the form of a straight tube and is con- 

 nected throughout its length with the body wall, the endothelial 

 tube is separated from the muscular tube by a marked layer of 

 delicate fibrils. In their papers upon the heart both His and 



4 A few data regarding all of the embryos described in this paper are given in 

 my catalogue of 500 specimens. Anat. Rec. ; vol. 5, 1911. 



