332 THE DEVELOPMENT OF THE CHICK 



The Laryngotracheal Groove.. The embryonic primordium 

 of the larynx and trachea communicates at first along its entire 

 length with the postbranchial division of the pharynx (72 hours). 

 At 96 hours the hinder portion of the groove is already converted 

 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 wahs branch- 

 ing into the two bronchi at its lower end. At its cephahc 

 end the lumen opens into a considerable cavity, representing 

 the larnyx; the glottis appears to be closed by a plug of 

 epithelial cells continuous with the sohd wall of the oesophagus. 

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

 closed by the thickened epithehum; 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 vacuoU- 

 zation; the lips of the glottis still show a complete epithelial 

 fusion. Thus it is apparent that the cavity of the larynx is es- 

 tabhshed by the formation of vacuoles within the soUd cell-mass, 

 and by their expansion and fusion. I cannot say how soon the 

 glottis becomes open. 



The development of the laryngotracheal apparatus, including 

 the cartilages and muscles, has not been specially investigated in 

 the chick. In general, it can be said that the parts external to the 

 epithehum arise from the mesenchyme, which begins 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 Wunderhch for Fringilla domestica that 

 the tympanic cartilage arises from the lower tracheal rings. The 

 origin of the musculature of the syrinx is not known. 



