186 JAY ARTHUR MYERS 



the cephalic border of the pessulus to the point of union of the 

 two bronchi. 



In a 226 hour embrj^o, the cells in the center of the anlagen 

 have no definite arrangement, but are larger and more o^'al in 

 shape than those in the preceding stages. The central cells, in 

 enlarging, have crowded the outer cells together, thus causing 

 the individual outer cells to appear flattened and the whole periph- 

 eral area to appear narrower. The first half-rings and the fourth 

 intermediate syiingeal cartilages are now separated by a greater 

 space than in the stages pre\dously described. The anlage of an 

 additional cartilage appear just cephalad to the developing fourth 

 intermediate syringeal cartilages. Though small, as yet, these 

 anlagen, the beginnings of the third intermediate syringeal car- 

 tilages, are fused \Aith the cephalic extremity of the ventral pyra- 

 mid of the pesulus. Dorsally these anlagen extend only slightly 

 more than one-half of the distance to the dorsal extremity of the 

 pessulus where they end unattached. These structures appeared 

 in only one out of two embryos taken at this stage. These anla- 

 gen are shown in figures 8 and 9, which make clear the relation 

 and the extent of these and other developing skeletal elements. 

 The first bronchial half-rings extend around the lateral walls of 

 the bronchi but do not reach the pessulus at either extremity. 

 The fourth intermediate syringeal cartilages arch cephalad to 

 some extent and are fused \nih. the ventral pyramidal extremity 

 of the pessulus. Their dorsal ends are unattached and lie some- 

 what lateral to the dorsal pyramid of the pessulus. The develop- 

 ing pessulus has gradually assumed the same form as in the 

 adult, except the shaft, which at this stage is round in cross sec- 

 tion, rather than triangular. At this stage, too, the mesenchyma 

 along the entire ventral surface of the trachea has become. more 

 condensed. This condensation represents the primordium of the 

 anlagen of the tracheal rings. 



The 248 hour stage shows a marked advance in the develop- 

 ment of the skeletal parts (fig. 14) All the tracheal rings are 

 represented by circular (in cross section) condensations of mes- 

 enchyma which extend around the lumen, immediately external 

 and in contact with the epithelial tube. The first and the second 



