468 WILLIAM A. LOCY AND OLOF LARSELL 



The obvious external feature of the tenth day is the indenta- 

 tion of the ribs on the dorso-lateral border of the lungs. Figure 

 19 shows a dissection exposing the right lung and adjacent or- 

 gans of an embryo at the beginning of the eleventh day. There 

 are four well marked indentations of the ribs. As mentioned 

 above, during the ninth and tenth days the lungs undergo a 

 change in position passing from a more ventral to a more dorsal 

 position, and in so doing come close against the ribs, and the 

 dorsal margin of the lung comes to lie along the vertebral column. 

 The air-sacs are enlarged and the proximal ends of the two pos- 

 terior ones are constricted to form a sort of neck. The abdomi- 

 nal air-sac has increased relatively faster than the others. 



The trunks of recurrent bronchi are also shown in connection 

 with the two posterior air-sacs. The recurrent bronchi are the 

 most important structural feature that we have yet had occasion 

 to mention. They begin on the ninth day as buds from the 

 proximal ends of the abdominal and the posterior intermediate 

 air-sacs, and, later, the other air-sacs, with the exception of the 

 cervical, g've rise to similar ourgrowths They are destined to 

 develop ramifications that anastomose with parabronchi in vari- 

 ous parts of the lung and play a very important part in its physi- 

 ology. They are so important that they receive separate treat- 

 ment in section 3 to which reference should be made for figures 

 and further details. 



Figure 20 is a diagram made from a study of the left lung of 

 an embryo, incubated 9^ days, to show especially the relations 

 of the mesial and the lateral moieties of the subbronchial sac at 

 this stage of development. In the preceding sketches (except 

 figure 18) the mesial moiety of the interclavicular sac has not 

 been shown chiefly because the sketches were executed before 

 we had learned to look for the two moieties of this sac, and, 

 further, the aspect from which the specimens were drawn did not 

 bring the mesial moiety into view. 



The diagram (fig. 20) was made from observations by re- 

 flected as well as by transmitted light and the connections of the 

 air-sacs with the bronchi are indicated. On the lateral border 

 of the lung is seen the lateral moiety {Lat.moi.) of the interclavic- 



