THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 555 



by HALLER, as auricular canal (ok). The atrium thereby acquires a 

 striking form, since its two lateral walls develop large out-pocketings 

 (ho), the auricles of the heart (auriculae cordis) ; the free edges of 

 the latter, which in addition soon acquire notches, are turned for- 

 ward, and subsequently enfold more and more the arterial part of the 

 heart, the truncus arteriosus (Ta), and a part of the surface of the 

 ventricle. 



The auricular canal (fig. 308 ok) is in embryos a well -distinguished 

 narrowed place in the cardiac tube. Owing to the great flattening 

 of its endothelial tube in the sagittal direction, its walls almost 



Fig. 306. 



Fig. 305. Heart of a human embryo, the body of which was 2'15 mm. long (embryo Lg), after 



His. [Compare fig. 313.] 

 K, Ventricle ; Ta, truncus arteriosua ; V, venous end of the S-shapei cardiac sac. 



Fig. 306. Heart of a human embryo that was 43 mm. long, neck measurement (embryo SI), 



after His. 

 , Ventricle ; Ta, truncus arteriosus ; ok, canalis auricularis ; vh, atrium with the heart-auricles 



ho (auriculae cordis). 



coming into contact, the passage between atrium and -ventricle is 

 reduced to a narrow transverse fissure. It is here that the atrio- 

 ventricular valves are afterwards developed. 



The fundament of the ventricle at first presents the form of a 

 curved tube (figs. 305, 306 k), which however soon changes its form. 

 For at a very early period there is observable on its anterior [ventral] 

 and posterior surfaces a shallow furrow running from above down- 

 ward, the sulcus interventricularis (fig. 307 si), which allows a left 

 and a right half of the ventricle to be distinguished externally. The 

 latter is the narrower, and is continued upward into the truncus 

 arteriosus (Ta), the beginning of which is somewhat enlarged and 



