466 WILLIAM A. LOCY AND OLOF LARSELL 



jection. As is well known, their blood supply in later stages is 

 derived from arterial branches coming from the aorta. The 

 superficial distribution of blood vessels is shown in figure 17 which 

 is from a specimen somewhat younger than that sketched in figure 

 16. It is to be understood that the internal plexi of capillaries 

 are extensive and the blood vessels represented in figure 17 are 

 those visible through the translucent walls of the lung and are 

 mentioned here merely as a feature of external anatomy. 



Figure 18 is a surface view of both lungs of an embryo at the 

 close of the ninth day of development. The bronchus of the 

 right side has been severed and the right lung rotated so as to 

 expose more fully the mesial facet. The five air-sacs are now 

 well projected beyond the lung wall and in this figure a new struc- 

 tural feature is brought into evidence. This is the mesial moiety 

 {Mes.moi.) of the interclavicular air-sac. At this stage it is con- 

 nected through the interclavicular canal with the anterior in- 

 termediate air-sac, and the mesial moiety is widely separated 

 from the lateral moiety. At a later period (fifteenth day, fig. 

 51) the mesial moiety comes into contact with the lateral moiety 

 and subsequently the two moieties fuse into one sac. In the 

 published sketches of surface views of embryonic stages (with the 

 exception of a figure by Selenka, '66) the mesial moiety has not 

 been represented. It is commonly hidden from view between 

 the two lungs. There are however some published sketches of 

 section of the lungs, as Lillie, '08, Juillet, '12, etc., in which it 

 has been represented as a forward projecting diverticulum of 

 the anterior intermediate air-sac, but in these sections it has 

 heretofore been interpreted as a portion of the anterior inter- 

 mediate sac. Also in a diagram of Juillet, '12, (cf. his fig. X, p. 

 313) the mesial moiety of the lung of the embryonic chick is 

 represented to the exclusion of the lateral moiety. In the genus 

 Larus, even in the adult, a separate lateral sac of the interclavicu- 

 lar is present in addition to the mesial portion of that sac (Juillet's 

 fig. XVII, p. 351). For the further history of these moieties of 

 the interclavicular air-sac see figures 47, 49, 50 and 52 and the 

 accompanying comments. 



