THE SYRINX OF GALLUS DOMESTICUS 193 



Immediately above the bifm*cation, the lumen of the tube is 

 at first slit-like in shape, but in later stages, 152 and 164 hours, it 

 is almost square in cross section. Its dorsal wall is about two 

 times as thick as its ventral wall. 



The developing cartilages produce slight evaginations of the 

 epithelium of the respiratory tube. In a 248 hour embryo these 

 evaginations occur along the entire length of the trachea and the 

 bronchi. 



At 332 hom-s only a single layer of cuboidal epithelial cells 

 covers the future tympanic membranes, while immediately ceph- 

 alad and caudad to these membranes two or three layers of 

 nuclei can be distinguished. A distinct layer of small nuclei 

 appears at the base of the epithehum. In the tympanic regions 

 this is the only layer of nuclei present. The cells possessing them 

 are slightly cuboidal, but at 452 hours they become very flat. 



Certain cells resembling goblet cells, and other cells with clear 

 cytoplasm, are present in the 332 hour embryo in all portions 

 of the epithelium, except that covering the tympanic membranes. 

 They are quite numerous in the semilunar membrane. At 452 

 hours certain groups of secreting cells are found which, when 

 stained with mucicarmine, and when traced through succeeding 

 stages, were determined to be simple mucous alveolar glands. 



Owing to the thickness of the sections it was difficult to detect 

 the "first appearance of cilia, but one can be reasonably sure of 

 their presence in 248 and 260 hour embryos. They are very 

 abundant in and following the 332 hour stage. 



Vacuoles in the epithelium appeared in the 284 hour stage, 

 but they were not very numerous in this embryo and are prac- 

 tically wanting in all other stages. 



The skeletal elements are first represented by a condensation 

 of the mesenchyma, which in a 152 hour embryo is most marked 

 ventrally in the region of the bifurcation. The anlagen of the 

 first bronchial half -rings appear just caudad to the bifurcation 

 in the lateral walls of the bronchi, and are the only skeletal ele- 

 ments represented in the 164 hour stage. The next anlagen to 

 appear are those of the fom-th intermediate syringeal cartilages 

 and of the pessulus which become detectable in 184 and 200 hour 



