SiQH A. 0. V. Tymms: 



III. — General Survey. 



Concurrent with the development of the trachea and Ijronchi 

 as hypoblastic outgrowths, the surrounding primitive mesoblast be- 

 gins to gradually condense, or concentrate, around them, and as 

 early as the end of the third day of incubation this concentration 

 is clearly indicated. As development proceeds, there are formed at 

 regular intervals in this denser mesoblast, concentrations of tissue, 

 each gradually assuming a ring-like form. 



Ultimately, by differentiation, they give rise to the more or less 

 cartilaginous frameAvork of the trachea and bronchi. However, \\\ 

 the region of the tracheo-bronchial junction, these ring-like con- 

 centrations are considerabl}" specialised, to form the supporting 

 structure of tiie syrinx. The most distinctive modiheation. and 

 one whose bolt-like form is so characteristic of the common fowl 

 is the appearance of a rod-like concentration just between the con- 

 joining lironchi, which expands into a dorsal and a ventral plate, 

 and hence, from its shape, has been well called the pessulus. As 

 the ring-like concentrations of the syrinx assume definition, thei'c 

 is a gradual eliange in I'egard to their relative size, some lieeoniing 

 strongly, otiiers poorly, developed, even to the extent of l)eeoming 

 mere vestiges. There is also a change in shape, some losing tlu'ir 

 circular form, and becoming flatter and flatter, until nierely l)an(l- 

 like rudiments. Still furtlier. thei-e is modification not only in 

 degree of development, but also in their mutual relations and le- 

 lation to the dorsal and ventral pessular plates. 



As the sup2)orting framework thus develops, the membranes gradu- 

 ally appear, and become more and more differentiated fi-oni the 

 surrounding tissue. The first to be indicated are the meml)ranae 

 externae, and. fi-oni the time when the tracheo-bronchial frame- 

 work begins to appear, the position of those membranes is recognised 

 by the gicater intejval between tlie last tracheal ring and the fii-st 

 bronchial semi-ring. In this interval the wall of the trachea gi-adu- 

 ally becomes deflected towards and into the lumen, while the tissue 

 subsequently changes in .structure, and finally gives rise to the very 

 thin oval-shaped meml)rane of the adult. The next membrane to 

 appear is the membrana semilunaris, which becomes thrown \\\^ as 

 a wedge-.shaped fold, cap|>ing the cranial border of the pessulus. 

 •It increases considerably in size, but in the adult it has become 

 relatively small. Along with the devel(»pment of this fold, the 

 inner- walls of the bronchi begin to be clearly separated from eae!i 

 othei- by a s[)aco, so as to give rise to the membranae internae. 'I'liis 



