294 .1. 0. V. Tywms: 



IV. — hccdop'uuj Elements at Different Stages. 



1. Nine days' incubation. — After nine daA-s of incubation, a 

 concentration begins to take place in the enveloping dense niesoblasi. 

 and this is the first indication of what will be the first bronchial 

 seiiii-i-ing. 



2. Ten days. — Early in the stage there has also appeared the first 

 indication of the last tracheal ring, but it is not so definitely marked 

 as the first bronchial has now become. Just between the uniting 

 bronchi, the mesoblast tissue is becoming very concentrated, and 

 from this the pessulus will develop later. Towards the close of the 

 tenth day, the penultimate ring and the secon<l bronchial semi-ring 

 are just forming, while the first bronchial semi-ring and the last 

 tracheal ring are now more clearly differentiated from the surround- 

 ing tissue. Between these latter two there is a considerable inter- 

 val, and in each case they cause the hypoblastic epithelium to project 

 somewhat into the lumen, the interval marking the site of the future 

 membrana externa. 



3. Eleven and twelve days. — In an early stage of an eleven days' 

 embryo, the pessulus begins to l)e indicated, as a change gradually 

 takes place in the I'ontre of the dense interbi'oncliial mesoblast. 

 Also, the developing tracheo-l)roiiehia] rings are becoming nioi'e 

 differentiated, and a little latei- in the eleventli day, and by the 

 twelfth day, the ante-])enultiiiiate fouith and fiftli-last rings have 

 just appeared, but ai'e inore marked m tlie dorsal i-egion. Also, 

 cranial to these, a largt' nunil)er of tracheal rings are developing, 

 while caudally the thiid l)ronehial semi-ring has appeared, and the 

 fourth is just beginning. The ])essular me.sctblast, in which the 

 pessulus is developing, now extends to both <loi-sal and ventral sui- 

 faces of the ti-acheal e.xtretnitv, wlieix' it is expanded to form dorsal 

 and ventral plates of dense; iiiesoi)Iast, fi-oni whii-h the triangular 

 cartilaginous plates will aiiso. (PI. XIX.. fig. 4. ) At this stage, thus, 

 the main elements of the su))]»orting framewoik are all indicated, but 

 while the fiist l)ronchial semi-ring and the last tracheal ring are now 

 large and clearly marked, the renuiinder are snuill, less definite, 

 and imperfect. The degree of development is seen by comparing 

 the extent to which each element has passed beyond the mid- 

 lateral region, towards the dorsal and ventral surfaces. The last 

 tracheal and first bronchial reach the dorsal and ventral surfaces, 

 where there is fusion with the pessidar plates of dense mesoblast, 

 except in the case of the ventral ends of the first bronchial, which 

 He free just caudal to the plate of mesoblast. This ventral plate 



