DEVELOPMENT OF FOURTH DAY 529 



venous atrium of the heart, while the distal portions remain as the jugu- 

 lar veins of the head region. 



The posterior cardinal veins (Figs. 301, 308), lie in the angle be- 

 tween the somites and the lateral mesoderm. It is of importance to 

 locate these vessels and understand their position, as the excretory sys- 

 tem develops in close relationship to them later, and their relation to 

 the excretory system cannot be understood unless their developmental 

 process is closely followed at this stage. 



The mesonephroi develop from the intermediate mesoderm so that 

 the posterior cardinal veins lie just dorsal to them throughout their 

 length (Fig-. 301). 



In fact, the posterior cardinal veins are the principal afferent ves- 

 sels of young embryos. However, in the adult these posterior cardinal 

 veins are going to be replaced by the large vena cava. 



With the foregoing in mind as a sort of general bird's-eye view of 

 what has taken place and what will take place in the main blood vessels, 

 we shall enter into a little more detail. 



THE HEART 



The heart began as a paired structure. When the ventral walls of 

 the embryo came together, the two portions of the heart also came to- 

 gether, and formed a single tube in the midline of the body, close to 

 the ventral portion. 



After this fusion, the heart is nearly straight and double-walled. 

 The endothelial lining of the heart has the same structure and is con- 

 tinuous with the entering- and outgoing blood-vesse 1 s. 



There is a thickened layer over the heart called the epimyocardium, 

 which later separates into a thickened muscular layer, the myocardium, 

 and a thin non-muscular covering called the epicardium. 



As the paired tubes have come together to form the single heart, 

 the splanchnic mesoderm from each side of the body has also come 

 together to form the dorsal and ventral mesocardia (Fig. 275). 



The ventral mesocardium disappears almost immediately after its 

 formation, but the dorsal mesocardium continues suspending the heart 

 for some little time, also disappearing ultimately, except at the more 

 caudal portion of the heart. 



As already described, the heart, now lying in the pericardial cavity, 

 is attached at both ends and grows much more rapidly than the sur- 

 rounding body, so that it begins to fold upon itself. The bending of 

 the organ must be carefully studied or later work upon the heart will 

 have little meaning. (Figs. 274, 276, 279, 280, 283, 287.) 



It will be noted that the cephalic end of the heart is attached just 

 where the aortae leave it, while the caudal end of the heart is attached 

 where the omphalomesenteric veins and the dorsal mesocardium meet. 



