p-^^p 



.y/f 



FK;. fi:iO. Development of the Frog. A F, segmentation ; G, overgrowth of ectodei-m ; H, I, 

 establishment of germinal layers; J, K, assumption of tadpole-form and establishment of 

 nervous system, notochord, and enteric canal ; L, newly-hatched tadpole, bl.ccel. blastocoale ; 

 Up. hip', blastoporc ; //r 1 . br-. gills ; br. d. branchial arches ; c. eye ; cct. ectoderm ; end. 

 endoderm ; ent. enteron ; /. br. fore-brain ; h. br. hind-brain ; m. br. mid-brain ; md. /. 

 medullary fold ; md. </>'. medullary groove ; mes. mesoderm ; mg. megameres ; mi. micromeres ; 

 nr/i. notochord ; n. e. c. neurciiteric canal; pcdm. proctodseum ; pty. pituitary invagina- 

 titdi ; ret. commencement of rectum ; sk. sucker ; sp. cd. spinal cord ; st.dm. stomodanun ; 

 t. tail ; ?/. yolk cells ; ?/A-. pi. yolk plug. (A D, F H, and J from Ziegler's models : 

 E, I, K, and L after Marshall.) 



