THE LVNiiS. 



409 



of the lungs. The smaller bronchi divide, for the most part, in 

 a regular, dichotomous manner, as shown on the right-hand 

 side of Fig. 163. The branching at first affects the hypo- 

 blastic lining alone, but about the thirteenth or fourteenth day 

 (Fig. 163) the mesoblastic wall becomes divided by external 



RO 



NY 



TO 



VD 



CH 



CR 



HI 



Rl 



PY 



CR 



RV 



CP 



FIG. 163.- A transverse section across the thorax of a Kabbit Embryo of the 

 sixteenth day. x 15. 



A, <lors;il aorta. CH, notocliord. CP, pericardia! cavity. CR, plenral cavity. 

 FN, ncnral arch of vertebra. LB, bronchus. LGr, luu.u. NS. spinal cord. NY, 

 sympathetic nerve cord. RA, right auricle. RB, left .auricle. RD, inter-miriciilar 

 sepruni. RI, rib. RW, capituluiu of rib. RO, tubercle of rib. RV. rif-lit ventricle. 

 RY. left ventricle. ST, ventnil end of rib. from \vliieli the sternum is formed. TO, 

 u'sophajjus. VD, left anterior vena cava. VI, posterior vena cava. 



grooves or clefts, which mark out the boundaries of the main 

 lobes of the lungs. The trachea, or median part of the air 

 passage, is at first very short ; but, as the neck elongates, and 

 the lungs get carried back into the thorax, the trachea rapidly 

 increases in length, and by the eighteenth day (cf. Fig. 151) 



