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Pig. 939. — Development of the Frog. A— F, segmentation ; G, ovorgrowth of ectoderm ; 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.cml. blastoccele ; 

 Up. hip', blastopore ; 6?'i. hv^. gills ; hr, cl. branchial arches ; e. eye ; ect. ectoderm, ; end. 

 endoderm ; ent. enteron ; /, hr. fore-brain ; h. hr. hind-brain ; m. hr. mid-brain ; md. f. 

 , medullary fold ; ond. gr. medullary groove ; 'mes. mesoderm ; ing. megameres ; trL micromeres ; 

 neh. notochord ; n. e. c. neurenteric canal ; pcdm. proctodseum ; pt}/. pituitary invagina- 

 tion ; ret. commencement of rectum; sk. sucker; sp. cd. spinal cord; st.dm. stomodseum ; 

 t. tail; yJc. yolk cells; yh pi. yolk plug. (A— D, F— H, and J from Zieglcr's modeLs : 

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



