THE EARLY DEVELOPMENT OF THE FROG 65 



FIG. 22. Representative stages in the development of the frog. C-F, after 

 Keibel(Kopsch). G, H, from Ziegler's models. A. Unfertilized ovum. B. Fully 

 formed gastrula; posterior view. C. Early stage in the formation of the central 

 nervous system; dorsal view. D. Side view of young embryo showing the rudi- 

 ments of the visceral arches. E. Side view of an embryo with central nervous 

 system established, and optic vesicles indicated. F. Side view of ''early embryo," 

 showing formation of head and tail regions. G. Side view of embryo just before 

 hatching. H. Fully formed tadpole, showing rudiments of hind-limbs, a, 

 Animal pole; b, blastopore containing yolk plug; ba, branchial arches; eg, rudi- 

 ments of external gills; hi, hind-limb bud; m, mouth; mh, mandibular and hyoid 

 arches; nf, neural folds; ng, neural groove; np, neural plate; o, olfactory and 

 stomodseal pit; op, rudiment of optic vesicle ; opr, right opening of opercular cavity, 

 just before its closure; p, proctodseum; pn, pronephric eminence; r, branchial 

 ridge (plate); s, oral sucker; t, rudiment of tail; tnf, transverse neural fold; 

 v, vegetal pole. 



