200 JAY ARTHUR MYERS 



axis of the trachea. The tracheal rings, the pessulus, and the 

 ventral ends of the first half-rings become ossified, while all other 

 skeletal parts remain cartilaginous. 



4. The external tympanic membranes appear between the 

 fourth intermediate syringeal cartilages and the first half-rings, 

 while the internal tympanic membranes extend from the caudal 

 borders of the pessulus to the bronchidesmus and represent merely 

 a modified part of the medial bronchial walls. 



5. The syrinx is hned with stratified ciliated columnar epithel- 

 ium containing numerous simple alveolar glands. Upon ap- 

 proaching the tympanic membranes this columnar epitheUum 

 is transformed into a stratified squamous epithelium which be- 

 comes a single layer of flattened cells over the membranes proper. 



6. The tympanum is attached to the remainder of the sjo-inx 

 only by elastic membranes. 



DEVELOFMENT 



1. The first indication of the respiratory system was observed 

 in a 68 hour embryo in which the laryngeotracheal groove and 

 the bronchi were represented. At first the trachea is much 

 shorter than the bronchi, but with the development of the neck, 

 it becomes, after the 140 hour stage, relatively much longer than 

 the bronchi. The walls of the trachea and the bronchi are at 

 first composed only of epithelium which contains two or three 

 rows of nuclei. 



2. The mesenchymal condensation common to the entire epi- 

 thelial tube first becomes markedly prominent at the tracheal 

 bifurcation in an smbryo of 152 hours. 



3. The anlagen of the first bronchial half-rings appear in a 176 

 hour embryo, those of the fourth intermediate syringeaL carti- 

 lages appear 12 hours later. The anlagen of the third intermedi- 

 ate syringeal cartilages and the anlage of the pessulus are pres- 

 ent at 200 hours. 



4. Distinct cartilage cells were first observed in the first bron- 

 chial half -rings. 



5. The first four tracheal rings have not united to form the 

 tympanum at hatching, nor have the other skeletal elements 



