252 



FRANKLIN P. MALL 



I mentioned above in describing the heart of the embryo 2 mm. 

 long that its endothelial lining is collapsed and suspended by a mass 

 of fine fibrils within the muscular tube. In slightly older stages 

 the arrangement of the endothelial lining is changing, becoming 

 dilated on the venous side of the heart. This change is beginning 

 in No. 3, is more advanced in No. 318 and is complete in No. 164 

 (fig. 2). His 3 has pointed out that the endothelial lining hugs 

 the muscle wall closely in the embryonic atrium, while it remains 

 suspended for a time in the rest of the heart. This arrangement 

 is so pronounced in the early heart that it affords a way by which 



V.. 



Fig. 2 From the reconstruction of a heart of an embryo 3.5 mm. long (No. 164). 

 X 66. View from the left side. 



we may determine with precision the exact portion of the heart 

 tube from which the atrium arises. My specimens show con- 

 clusively that the atrium arises exclusively from the free heart 

 tube and that the sinus venosus does not contribute to its forma- 

 tion. This being established it follows that as soon as the heart 

 tube is fully separated from the body walls that the anlage of the 

 entire adult heart is to be found between its arterial and venous 

 attachments. 



In the embryo 3.5 mm. long (No. 164) the completed heart 

 tube is seen, which is S-shaped and twisted upon itself so that the 



3 His, W., Anatomie mensch. Embryonen. Theil 3, Leipzig, 1885. 



