332 THE DEVELOPMENT OF THE CHICK 



into a tube lying beneath the anterior end of the oesophagus; this 

 is the beginning of the trachea; the anterior part of the original 

 groove represents the larynx, and its opening into the pharynx 

 the glottis. It is not clear whether the trachea arises as an out- 

 growth of the hinder end of the laryngotracheal groove, or from 

 the hinder portion of the groove itself, by constriction from the 

 pharynx. At 96 hours the lumen of the lower end of the trachea 

 and adjoining portion of the two bronchi is obliterated by thicken- 

 ing of the walls; this is, however, a very transitory condition. 



The growth of the trachea in length is extremely rapid, keep- 

 ing pace, of course, with the elongation of the neck. At six 

 days the trachea is a long epithelial tube with thick walls branch- 

 ing into the two bronchi at its lower end (cf. Fig. 191). At its 

 cephalic end the lumen opens into a considerable cavity, repre- 

 senting the larynx; the glottis appears to be closed by a plug of 

 epithelial cells continuous with the solid wall of the oesophagus. 

 At eight days the lumen of both larynx and glottis is completely 

 closed by the thickened epithelium; at eleven days the cavity 

 of the lower end of the larynx is re-established, and the 

 cell mass at the upper end is converted into a mesh-work by 

 vacuolization; the lips of the glottis still show a complete epi- 

 thelial fusion. Thus it is apparent that the cavity of the larynx 

 is established by the formation of vacuoles within the solid cell- 

 mass, and by their expansion and fusion. I cannot say how 

 soon the glottis becomes open. 



The development of the laryngotracheal apparatus, includ- 

 ing the cartilages and muscles, has not been specially investi- 

 gated in the chick. In general, it can be said that the parts 

 external to the epithelium arise from the mesenchyme which be- 

 gins to condense around the epithelial tube on the fifth day. 

 On the eighth day the glottis forms a decided projection into the 

 pharynx. Distinct cartilaginous rings in the trachea are not 

 visible on the eighth day, but are well formed on the eleventh 

 day. As regards the syrinx it has been established by Wunder- 

 lich for Fringilla domestica that the tympanic cartilage arises 

 from the lower tracheal rings. The origin of the musculature 

 of the syrinx is not known. 



