^96 A. 0. V. Tyinms: 



I'ruiii the fourtli lin^', so that it becomes like the sixth ring. The 

 fourth ling is still associated with the apex of the dense ventral 

 plate, which is becoming definitely triangular in shape. (PI. XIX., 

 fig. 5.) 



At a little later stage, on the thirteenth day, more changes are 

 apparent. The whole framework is of greater size, and more de- 

 hnite outline, and rapidly approaching tlie cartilaginous stage. 

 The boundaries of the pessulus are now well defined. Ventrally, the 

 framework now- presents its characteristic configuration — namely, 

 the last four tracheal rings are clearly involved in fusion with the 

 sides and apex of the ventral triangular plate, while its basal 

 angles articulate with the ends of the first l)ron(.hial semi-rings, and 

 with the latter the ends of the second l)i(>nchial have ])egun to fuse. 

 (PI. XIX., fig. 5.^ 



Dorsally, the ends of the ante-penidtimate have now extended to 

 the dorsal dense mesoblast. 



5. Fourteen days. — As before, there is increased size and dift'er- 

 entiation in the supporting framework, Avliile other developments 

 have appeared. On each side of the dorsal triangular plate lie 

 the free ends of the last tracheal, penultimate and first bronchial. 

 Just beyond the apex of the plate, there are the ends of the ante- 

 penultimate, which are noAv clearly defined and jjressed against each 

 other and the apex of the i)late. yet without fusing. The dorsal 

 ends of the fifth-last similarly come close togethei- in the middle 

 line, without fusing, but those of the sixth last do fuse, so tliat this 

 ling becomes the only complete one associated with the syrinx. The 

 fourth-last is still small, and now ])asses further dorsal than before. 

 Ventrally, no marked change has resulted. In addition to these 

 changt!S in the supporting framework, two other t'hanges are 

 aj)])arent. Firstly, the medial hypoblastic walls of the bronchi are 

 thrown into folds; and, secondly, the syringeal air sacs can now 

 be seen pushing their way up from the third fiitobronchus of the 

 lung tow^ards the interbronchial tissue. (PI. XVII., fig. I; and 

 PI. XXIV., figs. 19 and 24.^ 



As yet the membranae internae are not formed. The stem of 

 the interclavicular or syringeal air sacs has, however, divided into 

 its three main diverticula — namely, the large ventral sac. the dorsal 

 sac, and the smaller eraniallv-directed one, which, from the right 

 side, will become the sub-pessular sac. (PI. XXIV.. fig. 19.) 



'i'his latter change subseipiently becomes of great importance in its 

 bearing on the whole svrinx. and this fact lieeonies more and more 

 evident in following stages. 



