114 



THE BIOLOGY OF THE FROG 



CHAP. 



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

 teron; BR^,BR-^, DR'^; branchial arches; //i, Z/^, //3, gill slits; ///?, 

 hyoidslit; //J/, hyomandibular cleft; HY, hyo\d arch; /yV, infundibu- 

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

 duct. (From Morgan, after Marshall.) 



