Earliest Blood Vessels in Anterior Limb Buds. 



290 



about half-way between the seventeenth and eighteenth dorsal seg- 

 mentals. Thus two of the subclavian arteries on the right side are 

 true segmental vessels — those opposite the sixteenth and seventeenth 

 dorsal segmental arteries — but an equal number, two, are completely 

 out of harmony with the segmental plan. On the left side, the first 

 subclavian is opposite the seventeenth dorsal segmental vessels, the 

 second about midway between the seventeenth and eighteenth, and 



Spinal chord. //^ 



Myotome. 



PostCardV 



Umbilical Ve 





Fig. 8. — Cross section of chick of 33 somites iu region of anterior limbs and 

 midway between the sixteenth and seventeenth intersegmental vessels. The 

 section shows the fourth right and the second left subclavian. The section 

 shows clearly the peripheral limit of extension of the limb capillaries and the 

 nonvascular, marginal zone. 



U. v., umbilical vein. 



Post. Card. V., Posterior cardinal vein. 



the third subclavian considerably below the intersi)aeo belonging 

 to the eighteenth pair. Here, then, only one of the subclavians is 

 a segmental vessel, and o/ tlic total of seven vessels but three are 

 segmentally placed. The umbilical vein has been appreciably ex- 

 tended considerably below the region opposite the limb bud by a still 

 further caudal downgrowth of the primary body wall capillaries. 

 Opposite the limb, it receives on each side about ten distinct tribu- 

 taries, the upper ones, especially, being no longer capillaries in size 



