184 JAY ARTHUR MYERS 



with the lumen of the respiratory tract. Each cell appears light 

 and presents a pore-like opening which communicates with the 

 common pit. The nuclei of these secreting cells are situated 

 nearer the basement membrane than those of the typical epi- 

 thelial cells. These structures are not found in the epithelium, 

 covering the tympanic membranes. In a chick 24 hours after 

 hatching, such groups of secreting cells are much more numerous 

 and show a higher degree of development in having become more 

 invaginated. As determined by staining with mucicarmine it is 

 evident that these structures are the anlagen of the simple mu- 

 cous alveolar glands found in this region. Well developed cilia 

 project from the free borders of the typical columnar cells, but 

 they are not seen in the region of the tympanic membranes. 



The vacuoles, which occur in such great abundance in the 

 mucous membrane of the digestive tube in human embryos (John- 

 son '10), were observed in the respiratory tube of chick embryos, 

 but they are neither constant or abundant. In a 280 hour em- 

 bryo, buch vacuoles are quite numerous in the epithelium lining 

 the medial bronchial walls, but in all other stages they are prac- 

 tically absent. 



2. Cartilaginous skeleton 



Ir. an 80 hour embryo the epithelial tubes constituting the 

 trachea, the bronchi, and the oesophagus are all closely sur- 

 rounded throughout their entire extent by a common area of 

 mesenchyma which, in immediate relation to them, is much denser 

 than it is a little beyond them. An inspection of figures 12, 13, 

 and 7 will make clear this arrangement of the mesenchyma. 



In the 128 hour stage that portion of the condensed area of 

 mesenchyma surrounding the respiratory tract has become sepa- 

 rated from that surrounding the digestive tube. They now ap- 

 pear as two distinct areas, that surrounding the trachea being 

 the smaller. The condensed mesenchyma is, of course, sur- 

 roimded in turn by loose, unmodified mesenchymal tissue. 



In a 152 hour embryo the mesenchymal condensation, common 

 to the entire respiratory tract, becomes more proininent in the 

 region of the tracheal bifurcation than in any other portion. 



