240 THE CONTINUITY OF THE RACE 



modify embryonic organization and are variously utilized in the forma- 

 tion of subsequent adult structures. 1 The eustachian tube, for instance, 

 which connects our mouth cavity with the middle ear, is derived from 

 the first (foremost) gill pouch; and our lower jaw, the cartilages that 

 support our tongue, and our thyroid and parathyroid glands are deriva- 

 tives of various other gill arches or pouches. 



The limb buds. A fifth set of characteristic vertebrate structures, 

 the paired pectoral and pelvic limbs, also make their beginning in early 

 embryonic life. The paired buds that are to develop into the pectoral 

 girdle and forelimbs are the first to appear. They arise as lateral pro- 

 tuberances that show a very rapid growth, involving several of the paired 

 somites and associated tissues in the shoulder region. The buds that are 

 to form the hind limbs are markedly similar except for their more caudal 

 (tailward) position and their at first more retarded development. 



1 The aortic arches of the various vertebrate groups owe their arrangement to that 

 of the early embryonic gill arches, and our own pulmonary arteries and several of the 

 important branches from the basal part of our aorta are derived from the embryonic 

 blood vessels that traversed these arches. 



