THE DEVELOPMENT OF VEKTEBRATA. 



271 



and disappear soon.) But these six pairs are not all in 

 existence at any one time. The first appears early on the 

 second day, the sixth not until the fourth day, by which 

 time the first has vanished and the second is about to 

 follow. The fifth is so short-lived that for long it was 

 overlooked, and the sixth is called the fifth in older text- 

 books. 



With the great growth of lungs and liver after the fourth 

 day, other 

 important 

 changes occur. 

 The sixth aortic A 

 arch (fourth 

 branchial), as 

 in the frog, 

 sends* arteries 

 to the lungs ; 

 and veins from 

 the lungs run B 

 direct to the 

 heart. The 

 sub-intestinal 

 vein sends* 

 two series of 

 branches into 

 the mesoblast 

 of the liver 

 afferent and 

 efferent so 

 that a portal 

 circulation is 

 established. 



As this develops, the original main trunk of the sub 

 intestinal vein between these vessels fails to keep pace 

 with the growth of the rest of the vein ; and thus the 

 original sub-intestinal vein becomes separable into the 



* This convenient mode of expression is figurative. What actually 

 happens is that, as an organ grows, parts of its mesoblast are 

 differentiated into blood-vessels, and these become continuous with 

 pre-existing vessels. 



Fig. 143. DEVELOPMENT OF THE VASCULAR SYSTEM IN 

 THE BABBIT. 



Diagrammatic aide views. A and B will also serve for 

 the chick. 



