THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



233 



bulb. Before the aortic trunk and bulb are divided into the aortic arch and 

 pulmonary artery, four protuberances appear in the lumen (Fig. 204) . The 

 septum aorticum then divides the two which are opposite so that each vessel 

 receives three (Fig. 204). These then become concave on the side away from 

 the heart, in a manner which has not been fully determined, and at the same 



Atrial septum 



Right atrium 



Right atrio- 

 ventricular 

 (tricuspid) valves 



Right ventricle 



Ventricular 

 septum 



Pericardial cavity 





M5.\ 





-Hi! 



\ 





Dorsal aortic roots 



Amnion 



Upper limb bud 



Left atrium 



Left atrio- 

 ventricular 

 (bicuspid) valves 



Left ventricle 



Fig 206. — Transverse section of pig embryo of 14 mm. Photograph. 



time enlarge so that they close the lumen. Those in the pulmonary artery are 

 known as the valvules semilunares arteries pulmonalis, those in the aorta as the 

 valvules semilunares aorta. 



Changes after Birth.— The migratory changes of the heart from its origi- 

 nal position in the cervical region to its final position in the thorax will be con- 

 sidered in connection with the development of the pericardium (Chap. XIV). 

 With the exception of the septum atriorum, the heart acquires during fcetal life 

 practically the form and structure characteristic of the adult (Fig. 207) , So long 



