94 EDWARD MCCRADY, JR. 



The branchial pouches arise as successive local out- 

 pocketings of the branchial ridge from the anterior end 

 caudad. In the earliest specimen here shown (stage 25) the 

 first pouch is well developed and the second is distinctly recog- 

 nizable. In stage 26 in addition to these two a new out- 

 pocketing is seen, which is labeled posterior branchial com- 

 plex, and which represents the fused anlagen of the third, 

 fourth, and fifth gill pouches. As the third is separating from 

 this complex in stage 27, a still more caudal pouch (l.p.) in 

 this same ridge forms the anlage of the left lung. The line 

 across it in the figure shows the location of the cut end of the 

 model seen in figure 32, A. 



At this stage the atria of the heart tubes are still separate 

 and the deep ventral groove in the pharynx lies between them. 

 On first sight this groove appears to be the laryngo-tracheal 

 rudiment, but an examination of the late specimens of stage 27 

 shows this not to be the case. For when the atria come to- 

 gether in the midline (fig. 32, B, and fig. 33, stage 27+) this 

 groove is completely eliminated and the floor of the pharynx 

 is pushed up to the level of the lung pouches. Meanwhile, the 

 roof of the pharynx, which was depressed by the central 

 nervous system in stage 25 (see also fig. 31, second branchial 

 pouches), has lifted dorsally as the central nervous system 

 moved away from it in connection with the cervical flexure, 

 so that now the lungs are at the ventro-lateral corners of a 

 new triangle. In stage 28 the lung buds and the branchial 

 ridges begin to pinch off from the more dorsal part of the 

 pharynx from behind forward, so that in stage 29 a distinct 

 trachea is formed from the fusion of the posterior portions 

 of the original lateral branchial ridges. This is in keeping 

 with the fact that the cartilages of the trachea, which will 

 form in stage 34, are generally regarded to be of branchial 

 origin. 



Fig. 30 Reconstructions and sections of stage 27. The central reconstruction 

 is from 17141 as are also sections G, F and H. Sections A and B are from 17142, 

 and sections 0, D and E and reconstruction t are from 17154. 



