1 68 THE ENTODERMAL CANAL AND THE BODY CAVITIES 



At the end of the sixth week the cartilaginous skeleton of the larynx is indicated by 

 surrounding condensations of mesenchyme. The cartilage of the epiglottis appears rela- 

 tively late. The thyreoid cartilage is formed as two lateral plates, each of which has two 

 centers of chondrification. These plates grow ventrad and fuse in the median plane. 



The anlages of the cricoid and arytenoid cartilages are at first continuous. Later, sepa- 

 rate cartilage centers develop for the arytenoids. The cricoid is at first incomplete dorsad, 

 but eventually forms a complete ring. The cricoid may therefore be regarded as a modi- 

 fied tracheal ring. The corniculate cartilages represent separated portions of the aryte- 

 noids. The cuneiform cartilages are derived from the cartilage of the epiglottis. 



The Trachea. This gradually elongates during development and its 

 columnar epithelium becomes ciliated. Muscle fibers and the anlages 

 of the cartilaginous rings appear at 17 mm. The glands develop as 

 ingrowths of the epithelium during the last five months of fetal life. 



FIG. 174. Ventral and dorsal views of the lungs from a human embryo of about 9 mm. 

 (after Merkel). Ap., Apical bronchus; Di, D2, etc., dorsal, Vi. Vj, ventral bronchi; 

 Jc. infracardiac bronchus. 



The Lungs. Soon after the lung anlages, or stem buds, are formed 

 (in 5 mm. embryos), the right bronchial bud becomes larger and is directed 

 more caudally (Fig. 171). At 7 mm. the stem bronchi give rise to two 

 bronchial buds on the right side, to one on the left. The smaller bronchial 

 bud on the right side is the apical bud. The right and left chief buds, 

 known as ventral bronchi, soon bifurcate. There are thus formed three 

 bronchial rami on the right side two on the left, and these correspond to 

 the primitive lobes of the lungs (Figs. 168 and 174). 



On the left side, an apical bud is interpreted as being derived from the first ventral 

 bronchus (Fig. 174). It develops later and remains small so that a lobe corresponding to 

 the upper lobe of the right lung is not developed (Narath). The upper lobe of the left 

 lung thus would correspond to the upper and middle lobes of the right lung. 



The bronchial anlages continue to branch in such a way that the stem 

 bud is retained as the main bronchial stem (Fig. 1 74). That is, the branch- 

 ing is monopodial, not dichotomous, lateral buds being given off from the 

 stem bud proximal to its growing tip. Only in the later stages of develop- 



