DEVELOPMENT OF THE HUMAN HEART 



253 



arterial and venous ends are brought close together. At the venous 

 end the muscle wall is slightly dilated which marks the atrium; 

 this is lined closely with endothelium which encircles the cavity 

 within. No delicate fibrils are here seen between the muscle wall 

 and its endothelial lining. Then follows an upper bend to the 

 heart after which there is a dilatation projecting towards the left 

 side, the former marking the atrial canal and the latter the left 

 ventricle. The lower connecting piece unites the left ventricle 

 with the bulb which later on gives rise to the right ventricle. In 

 the atrial canal (Haller's auricular canal) the endothelial tube is 

 seen as a solid strand of cells suspended freely in the muscle wall 



Fig. 3 Section of the heart of the embryo 3.5 mm. long. X 66. 



by the delicate fibrils already mentioned. In the left ventricle 

 the tube shows a distinct cavity, while throughout the rest of the 

 heart tube the cavity is irregular but not pronounced. The form 

 of the endothelial tube is shown in fig. 2 and again in a semidia- 

 grammatic figure of a transverse section through the atrium, ven- 

 tricle and bulb in fig. 3. The delicate fibrils, which no doubt 

 belong to the endothelial cells are present in large number through- 

 out the whole heart tube, excepting in that which forms the atrium. 

 In another embryo (No. 384, 2 mm. long), considerably smaller 

 than the one just described and probably pathological, the degree 

 of development of the heart is practically identical with the one 

 3 h mm. long (No. 1(34). 



