188 JAY ARTHUR MYERS 



third. At this stage, the first are not fused with the ventral 

 pyramid, but are continuous with a mass of condensed mesen- 

 chyma which may be said to be a part of the apex of the pyi'a- 

 mid. They pass around the trachea and end freely on its dorso- 

 lateral aspect. 



The first two tracheal rings are very small and, like the first 

 three intermediate cartilages, are composed entirely of con- 

 densed mesenchyma. At both their dorsal and their ventral 

 extremities they present a marked caudal dipping The second 

 tracheal ring is, however, somewhat larger and more advanced 

 in its development than the first. It should be noted, also, that 

 the first four tracheal rings are not in direct contact with each 

 other as in the adult ; but are separated by spaces which are occu- 

 pied by embryonal fibrous tissue. The remaining tracheal rings 

 call for no special description, since they differ from those of the 

 adult only in size, in being circular in cross section, and in being 

 composed entirely of cartilage. The ventral and dorsal extremi- 

 ties, or pyramids, of the pessulus, are cartilaginous throughout 

 and are similar in shape to these parts in the adult. The shaft, 

 however, as stated above, is round in cross section. 



Second to the pessulus in size are the first bronchial half-rings. 

 These half-rings in their middle portions, arch markedly caudad. 

 Their ventral extremities are in close contact mth the base of 

 the ventral pyramid of the pessulus, but fusion has not taken place. 

 Dorsally they end unattached, caudad and lateral to the pessulus. 

 The second half-rings, which are much smaller and immediately 

 caudad to the first, are connected with the latter at their ventral 

 extremities by means of a small mass of condensed mesenchyma. 

 A wax reconstruction of the syrinx at this stage (284 hours) 

 shows that the lower end of the trachea exhibits a slight compres- 

 sion (fig. 10). In the region of the intermediate syringeal carti- 

 lages this compression is well marked. 



In a 320 hour embryo, all the skeletal elements are represented 

 by primary cartilage. The first and the second intermediate 

 syringeal cartilages are the last to be transformed into true car- 

 tilages, and at this stage are the smallest of all the skeletal 



