456 WILLIAM A. LOCY AND OLOF LARSELL 



providing material for blootl vessels, lymph spaces, muscles, con- 

 nective tissue and like elements, while ingrowths from the ecto- 

 derm provide the nerve supply. 



In front of the bulges the walls of the pharynx are compressed 

 laterally and the tube is narrowed on its ventral border to form 

 the laryngo-tracheal groove which is the forerunner of trachea 

 and larynx. 



Figure 3, showing the pharynx and the ventral groove, is a 

 camera tracing of two consecutive sections taken 120 and 128 

 microns in front of the one sketched in figure 2. In still earlier 

 stages the narrowing of the respiratory portion of the pharynx 

 is easily seen as well as the incipient stages of the bulges from 

 which the lung pockets are produced. 



The cavity of the pharynx is also narrowed just above the 

 lung pouches (fig. 4) so that in cross section, the outline is simi- 

 lar to that of figure 3 in an inverted position. 



Immediately the lung pouches begin to elongate by growth of 

 the endodermal lining in a caudo-lateral and somewhat dorsal 

 direction, and prior to the (30th hour, their divergent distal ends 

 become separated from the oesophagus (fig. 4). By the end of 

 the third day (72 hours) the embryonic lung can readily be ex- 

 posed by dissection. 



At the close of the fourth day (96-hour stage) the lungs and 

 adjacent territory present the appearance shown in figure 5. At 

 this stage the lungs are small, smooth pouches extending cau- 

 dally and dorsally along each side of the oesophagus. In this 

 specimen, the sixth arch, from which the proximal end of the 

 pulmonary artery is shortly to develop, has not been completed 

 although a ventral spur of the arch is shown and a shorter dorsal 

 spur from the aorta. 



Figure 6 represents a ventral view and figure 7 a lateral view 

 of the lung of a chick embryo near the close of the fourth day 

 of development. The lung pouches are divergent and their dis- 

 tal portions extend caudad, laterad and dorsad. Their cavities 

 are lined by the endodermal diverticula from the pharynx, and 

 these are surrounded by mesoderm, so that the surface exposed 

 by dissection is mesodermic. The walls of the endodermal tube 



