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bulletin: museum of comparative zoology. 



blood is conducted iu the main through what is obviously the persistent 

 azjgos (v^n. az.), though remnants of the hemiazygos occur between the 

 ninth and teutli right intercostals, as well as between the eleventh and 

 twelfth. 



Thus in the three types considered the anterior part of the system is 

 always formed exclusively from the hemiazygos. The posterior part may 



A. B. C. 



FlGUBE 4. 



be derived from the equally persistent azygos and hemiazygos (type one) 

 or from a preponderant hemiazygos (type two) or finally from a pre- 

 ponderant azygos (type three). 



Fig. 4. The hemiazygoB and connected veins, showing three structural types. 

 Raw starch injections. Ventral views, vn. az., azygos vein; vn. hm'az., hemi- 

 azygos vein ; 6, 10, 13, bases of the sixth, tenth, and thirteenth intercostal veins 

 respectively. 



A. Type one, with equally persistent azygos and hemiazygos. From an em- 

 bryonic pig about seven centimeters long. X 3. 



B. Type two, with preponderant hemiazygos. From an embryonic pig about 

 eleven centimeters long. X 2. 



C. Type three, with preponderant azygos. From an embryonic pig about 

 twenty centimeters long. Natural size. 



