250 FRANKLIN P. MALL 



The points to be discussed will be considered in the following 

 order: A, Subdivisions of the early heart; B, Formation of the 

 septum and atrio-ventricular valves; C, The atrio- ventricular 

 bundle; D, Musculature of the left ventricle. 



A. SUBDIVISIONS OF THE PRIMARY HEART TUBE 



In an embryo about 2 mm. long (No. 391), which was modeled 

 in wax by Dr. Dandy, the heart is shown as a relatively straight 

 tube with its arterial end directed towards the head (fig. 1). 

 Its muscle wall is of even thickness and communicates throughout 

 its whole length along the dorsal midline with the rest of the meso- 

 derm, that is it has a complete dorsal mesentery. 2 At its anterior 

 end the heart tube shows a slight dilatation just before the arte- 

 ries arise from it. Midway between the two ends of the muscle 

 tube there is an indentation on the left side which marks the begin- 

 ning of the bulbo-ventricular groove, that is, it separates the 

 atria and left ventricle on the one hand from the right ventricle 

 and the bulb on the other. Within the muscle tube there is 

 suspended by means of numerous fine fibrils the collapsed endo- 

 cardial lining. These fibrils will be considered later when the 

 development of the valves is discussed. 



The heart now separates rapidly from the rest of the mesoderm 

 in subsequent stages and is soon suspended in the pericardial 

 coelom, remaining attached to the body only at its venous and 

 arterial ends. The indentation on the left side of the heart, 

 mentioned above, becomes more pronounced, the heart rolls 

 upon itself quite rapidly as succeeding stages of development 

 show. In an embryo 2.5 mm. long (No. 3) the heart is separated 

 from the dorsal midline in its middle, while in another of the same 

 length (318) the separation is more pronounced. In one 3.5 mm. 

 long (164) the separation is complete and the various subdivisions 

 of the final heart tube can be outlined with precision (fig. 2). 



2 A complete description of this embryo is given by Dandy. Amer. Jour. Anat., 

 vol. 10, 1910. Sections through the heart are shown in his figs. 1 and 2. Evans 

 (Keibel-Mall, Manual of human embryology, vol. 2, fig. 409). also pictures sections 

 through the heart of this embryo. Also by Mall, Ibid., vol. 1, figs. 382-386. 



