EARLY DEVELOPMENT OF THE HEART AND PAIRED BLOOD-VESSELS 257 



and myocardium (muscle layer of heart). This type of heart occurs in human em- 

 bryos of 2 mm. and 5 and 6 somites (Fig. 249) and shows three regions: (1) the 

 atrium, which receives the blood from the primitive veins, (2) the ventricle, (3) 

 the bulb, from which is given off the ventral aorta. 



As the cardiac tube grows faster than the pericardial cavity in which it lies 



it bends to the right, the bulbus 

 ,-: x and ventricle forming a U-shaped 



loop (Fig. 250, A, B). Four regions 

 may now be distinguished: (1) the 

 sinus venosus; (2) the atrium, also 

 thin walled and lying cranial to 

 the sinus; (3) the thick-walled ven- 

 tricular limb, ventrad and caudad in 

 position; (4) the bulbar limb, cranial 



'ero 



Fig. 248. — Diagrams to illustrate the origin of the 

 tubular heart (Strahl and Carius, from McMurrich's 

 " Development of Human Body"), am, amnion; en, 

 entoderm; h, heart; i, digestive tract. 



Fig. 249. — The heart of a 2 mm. human em- 

 bryo in ventral view (Mall). 



to the ventricular limb and separated from it by the bulbo-ventricular cleft. 

 Next in embryos of 3 to 4 mm. the bulbo-ventricular loop shifts its position until 

 its base is directed caudad and ventrad (Fig. 250, B). At the same time the sinus 

 venosus is brought dorsal to the atrium, which in turn is cranial with relation 

 to the bulbo-ventricular loop, and the bulbar limb is pressed against the ventral 

 surface of the atrium and constricts it. 

 17 



