syrinx oj Foai. 295 



ulsu rcLcivcs the ventral eiifls ul" the seeoiid and third-last rings, 

 while just beyund them are the ventral ends of the fourth, fifth and 

 sixth-hist rings. Those of the fourth and fifth fuse on eacli side, 

 and tlien with those of the other side, while those of the sixth fuse 

 in the mid-line, and lie quite separate from the others. As to the 

 t'Xtent of dorsal development, the fourth and fifth-last scarcely pass 

 ihe mid-lateral line before fading away, and then, likewise, a little 

 further, the third and the sixth-last. Still further dorsally, the 

 .second-last fades away, as well as the remnants of the bronchial 

 semi-rings, except the first, which, together with the last tracheal 

 ring, passes right to the dorsal surface, there merging into the 

 pessular mesoblast plate. Even at this early stage, it is noticealjle 

 that the ventral development is much less restricted than that of 

 the dorsal. 



4. Thirteen days. — By this stage a considerable number of 

 l)ronchial semi-rings and tracheal rings, not directly concerned with 

 the syrinx, have begun to form. Those concerned with the syrinx 

 have all become more distinctive, especially the last two tracheal 

 rings, and the first two bronchial semi-rings. This is seen in re- 

 gard to their structure, size and outline. The interval between the 

 last tracheal and first bronchial is greater, and hence the rudinjents 

 of the membranae externae are more marked. The difference in size 

 is even now very noticeable, the third, fourth and fifth last being 

 relatively verv small. The more distinctive structure and outline 

 is seen, not only in the rings, but also in the pessulus and its plates, 

 concentric circles of cells are now seen surrounding the pessulus. 

 As to the greater extent of developjiient, compared with the last 

 .stage, this is seen in a few particulars. In the dorsal region, tha 

 extremities of the last tracheal ring and the first bronchial semi- 

 ring are now being clearly differentiated from the dense dorsal 

 me.soblast. so that their ends lie somewhat free. (PI. XX., fig. !l.) 



As early as this stage a variation is at times apparent. Tjie 

 fourth-last ring, which is most poorly developed, and passes little 

 towards the dorsal surface, usually fuses on the left side with the 

 third la.st, but sometimes, though less often, the fusion takes pla.-e 

 with the fifth ring. This fusion on the left side is indicated at this 

 stage, while on the right the ends are quite free. As in the last 

 stage, there is little dorso-lateral development of this ring. As for 

 the second, fifth and sixth-last rings, they now reach the dorsal sur- 

 face Avhere they merge into the dorsal plate of mesoblast, from Avhich 

 the dorsal triangular plate is not yet clearly defined. The chief 

 changes in the ventral region are the separation of the fifth riii" 



