142 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



spaces. This system of longitudinal anastomosing vessels may be traced 

 through at least the whole length of the thorax, and lies only a little 

 dorsal to the region occupied by the hemiazygos, being thus in the 

 neighborhood of the forming vertebral column. It seems to us highly 

 probable that these anastomosing vessels were the ones seen by Rathke, 

 and supposed by him to enter partially into the formation of the azygos 

 and hemiazygos veins, a conclusion which, though in our opinion errone- 

 ous, is nevertheless not surprising when one considers the methods of 

 investigation employed in his time. What the origin of these vessels 

 may have been and what their subsequent fate may be have not been de- 

 termined by us, but that they contribute nothing to the formation of 

 the azygos or of the hemiazygos we feel perfectly assured. 



The development of the azygos and hemiazygos veins in swine con- 

 forms, then, in general to that found by Hochstetter ('93) and by Zum- 

 stein ('96, '97) in other mammals. 



In one respect, however, there is in this connection still ground for 

 difference of opinion among recent students ; this has to do with the 

 proportions in which the postcardinal and accessory components enter 

 into the formation of the azygos and hemiazygos. According to Zum- 

 stein ('96, p. 601), in the human being the azygos and the hemiazygos, 

 if there be one, are formed from the postcardinals exclusively, whereas 

 in the Guinea pig (Zumstein, "97, p. 188) both vessels are almost entirely 

 formed from the accessory veins. In the rabbit, and probably also in 

 the cat, according to Hochstetter ('93, pp. 589 and 595), the plane of 

 separation between the two components lies in the eighth thoracic seg- 

 ment, and this agrees very nearly with our observations on swine, where 

 it lies near the tenth pair of ribs. These differences, which are clearly 

 not fundamental, are more likely due to peculiarities in the develop- 

 ment of the respective species than to errors of observation, though such 

 topographical determinations are by no means simple. 



Conclusions. 



1. Small embryonic pigs possess well developed right and left post- 

 cardinals (posterior cardinal veins), which extend from the bases of the 

 corresponding posterior extremities anteriorly over the dorsal surfaces 

 of the mesonephroi to the Cuvierian ducts. 



2. The thoracic portion of each postcardinal persists from the heart 

 to the region of the tenth pair of ribs, beyond which a new vessel, the 

 accessory vein, is developed to a point some distance posterior to the 

 last pair of ribs. 



