THE RESPIRATORY ORGANS 63 



terminal dilatations of the mesobronchus or the 

 secondary bronchi. It is generally stated that 

 the abdominal sac is the first to appear, and is 

 early recognisable as a dilatation at the end 

 of the mesobronchus. The caudal thoracic 

 sac also arises from the mesobronchus, but not 

 until after the seventh day. The cervical, 

 clavicular, and cranial thoracic sacs begin to 

 grow out from secondary bronchi on the fifth, 

 sixth, and seventh days respectively. The 

 axillary sac is a prolongation of the clavicular 

 sac, and so has never an independent con- 

 nection with bronchi. 



It is to be noted that the cavities of the 

 embryonic bones, which afterwards become 

 pneumatic, are filled with marrow in the 

 embryo. Their invasion by the air-sacs is 

 a late development ; Selenka, for example, 

 stating that it only occurs in the humerus after 

 the twenty -second day, when the bone is 

 nearly full grown. 



