244 Professor Clark on a Case 



fifth branchial pairs. The anterior and posterior aortas are still 

 connected by a branch of communication, the remains of the 

 original root of the aorta. When the lungs are formed, the 

 beginning of the descending aorta supplies them with blood. 

 Its contents are in fact entirely distributed to them, with the 

 exception of what still passes through one remaining branch of 

 communication with the root of the aorta on the left side, 

 which is the ductus arteriosus : so that now the descending- 

 aorta can only receive its blood from its branch of commu- 

 nication with the anterior aorta. Hence the branch of com- 

 munication increases, and forms between the anterior and pos- 

 terior principal trunks the arch of the descending aorta. This 

 now derives its blood entirely from the left side of the heart, 

 and becomes the common origin of the ascending and descending- 

 vessels : whilst that which was its original stem, and which arose 

 from the right side, now becomes the pulmonary artery. 



Thus the aorta is formed from the concourse of branchial 

 arteries which spring from the heart, and whose direction seems 

 to be determined by the situation of the central nervous masses, 

 for they stretch towards the rudiments of the brain and then 

 descend in the neighbourhood of the spinal cord. 



From this point the completion of the embryo requires the 

 co-operation of at least two of the original elements of the ger- 

 minative membrane. 



The liver is formed by two productions of the upper part of 

 the intestinal tube. They proceed from the intestine as cylin- 

 drical prolongations of the mucous membrane surrounded by their 

 vascular layer : these subdivide, and form many arborescent hollow 

 filaments. Thus are constituted two principal lobes, which 

 surround the ascending vein to the heart. This vein sends off 



