140 bulletin: museum of comparative zoology. 



that in embryonic sheep, in addition to a postcava, an azygos vein also 

 occurred. Eathke seems to have become cognizant of this fact, for in 

 his third paper on this subject he abandoned his earlier views on the 

 fate of the postcardinals, and, without stating reasons for the change, 

 adopted views more nearly in accordance with the new observations 

 made by Stark. 



According to Rathke's ('38, pp. 3, 4, and pp. 10, 11) later view, the 

 anterior thoracic portion of the left postcardinal is involved in the for- 

 mation of the hemiazygos, and the right postcardinal in a similar way 

 enters into the formation, not of the postcava, but of the embryonic 

 azygos. This view agrees in the main with the results obtained by recent 

 investigators (Hochstetter, '93, and Zumstein, '96, '97) on other mam- 

 mals, and is abundantly confirmed by our own observations on swine. 



The facts thus far stated are only a partial exposition of Rathke's later 

 opinion. Only the anterior portions of the azygos and hemiazygos 

 are formed, according to Rathke's later view, from the postcardinals, 

 the posterior parts being developed from a system of longitudinal 

 anastomosing trunks between the successive intercostal veins. These 

 anastomosing trunks receive the blood from the intercostals, pass- 

 ing it forward towards the heart, and thus form a longitudinal ves- 

 sel, which gradually replaces a part of the original postcardinal. The 

 extent to which this replacement occurs may be indicated as follows. 

 The part of the hemiazygos extending from near the heart to the sixth 

 intercostal vein represents a persistent part of the postcardinal, and the 

 remaining part from the sixth intercostal posteriorly to the last one is a 

 new formation from the longitudinal anastomosing vessel ; the part of the 

 embryonic azygos from its connection with the heart posteriorly to the 

 eighth or tenth intercostal represents the right postcardinal, the remain- 

 ing posterior portion having been derived from the longitudinal anas- 

 tomosing vessel of that side. After the right intercostals establish 

 transverse connections with the hemiazygos, the azygos disappears, thus 

 leaving the hemiazygos as a return trunk for the blood from the right 

 as well as from the left intercostals. 



That the posterior portions of the azygos and of the hemiazygos in 

 many mammals are new formations added to the remnants of the post- 

 cardinals is now, we believe, generally admitted, and, as we ourselves 

 have seen, is certainly true for swine ; but that these new formations, 

 the accessory veins, develop from anastomosing branches between 

 the intercostal veins, as stated by Eathke, is not, we believe, in accord- 

 ance with fact. Of the several embryos examined by us at the stage in 



