194 JAY ARTHUR MYERS 



embryos respectively. The third intermediate syringeal carti- 

 lages are represented at 226 hours. All of the tracheal rings, the 

 intermediate cartilages, and the first two half-rings are repre- 

 sented in the 248 hour embryo. 



Cartilage cells were first observed in the 284 hour embryo, in 

 the first half-rings, in the fourth intermediate cartilages, and in 

 the pessulus. The first tracheal ring and the first and the sec- 

 ond intermediate syringeal cartilages are the last to differentiate. 

 Differentiation for these cartilages begins in the 320 hour embryo. 

 No bone has developed at the time of hatching, nor has the tym- 

 panum been fully formed. 



The submucosa begins to develop ag 284 hours. At first it is 

 represented by mesenchymal cells, which later produce white and 

 elastic fibers. These fibers, however, do not take Mallory's 

 connective tissue stain until the 404 hour stage. At this time 

 the submucosa contains numerous nerves and blood vessels. 



The two sterno-tracheal muscles are quite well differentiated 

 at 176 hours. At 188 hours the long cytoplasmic strands or 

 processes of the developing cells are collected into bundle-like 

 masses. Faint cross striations were first observed in a 296 hour 

 stage. At 452 hours the muscles are not essentially different in 

 structure from those of the adult. 



The intercartilaginous membranes in early stages are not essen- 

 tially different in structure from the submucosa. In a 260 hour 

 embryo, the cells nearest their external surfaces elongate and 

 later develop strands of connective tissue fibers which extend be- 

 tween the cartilages and attach to these membranes. Promi- 

 nent bundles of such fibers extend caudally from the pessulus 

 and enter into the formation of the medial walls of the bronchi, 

 in a 404 hour embryo. 



In early stages the external and the internal tympanic mem- 

 branes do not differ in structure from the smaller intercartilagi- 

 nous membranes of the trachea and the bronchi. At the time 

 of hatching the tympanic membranes are quite thick. It is not 

 until after this time that they become thinner and appear as 

 true vibrating membranes. 



