ii 4 



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; HM, hyomandibular cleft; J/Y, hyoid arch; IN, infundibu- 

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

 duct. (From Morgan, alter Marshall.) 



