14 



THE BIOLOGY OF THE FROG 



Fig. 29. — Horizontal section through an advanced embryo. AR, archen- 

 teron; BR^,BR-^, BR'^, branchial arches; H^, H"^, H^, g\\\ slits; HB, 

 hyoidslit; //.T/, hyomandibular cleft; //F, hyoid arch; /AMnfundibu- 

 lum ; OF, olfactory pit ; OS, optic stalk ; P, pronephros ; 6", segmental 

 duct. (From Morgan, after Marshall.) 



