304 A. 0. V. T>jmms: 



Fig. 1. — Stage of 13-14 days' incubation. 



Fig. 2. — Stage of 15 days' incubation; interbioncliial air sacs ui 

 position, lateral sacs appearing. 



Fig. o. — Stage of hatched condition; all the syringeal air sacs 

 in position, and the Avhole structure rapidly approaching 

 the adult condition. 



Figs. 4 to JO. — Frontal sections through the ventral and dorsal tri- 

 angular plates of the pessulus at different stages, showing 

 their relation to the tracheal rings, and the first bronchial 

 semi-ring. 



Fig. 4. — Stage of 11-12 days' incubation, ventral triangular plate 

 appearing, along with the four last tracheal rings, and first 

 bronchial semi-ring. 



Fig. 5. — Stage of 13-14 days' incubation, ventral plate now well 

 formed, with the four last rings fused to it. First bronchial 

 semi-ring articulates, but is not fusing with the plate. 



Fig. 6. — Stage of 15 days' incubation. The ventral ends of the 

 third-last tracheal ring are separated from the ventral plate. 

 The second bronchial semi-ring is fused ventrally with the 

 first bronchial semi-ring. 



Fig. 7. — Stage of 16 days' incubation. The right ventral ends of 

 the second-last tracheal ring are now separated from the 

 plate, as well as the ventral ends of the third. 



Fig. 8. — Stage of the hatched condition. The left ventral end of 

 the third-last ring is separated from the plate, so that only 

 the last tracheal ring has its ventral ends fused to the ven- 

 tral plate. 



Fig. I). — Stage of 13-14 da^^s. Dorsal triangular plate appearing. 



Fig. 10. — Stage of 15 days. Dorsal plate well formed, Avith the 

 various tracheal rings and first bronchial semi-ring articu- 

 lating, but not fusintr, with it. 



Figs. 11-iG. — Stage of 16 days' incubation. Frontal, or longitudi- 

 nal horizontal sections, through the region of the tracheo- 

 bronchial junction, to show the arrangement and origin of 

 the syringeal air sacs, derived from the interclavicular sac. 



Fig. 11. — The interclavicular sac is seen passing up from its com- 

 mon stem of origin with the anterior thoraiic air sac into 

 the interbronchial region, and opening ventrally at the 

 cranial end of the bronchus, into the huge vciitro-latt'ral 

 sac, situated behind the tiiangular plate, ami latcial to the 

 lowci' tracheal iin<rs. 



