XIII 



PHYLUM CHORDATA 



281 



FIG. 950. Development of the Prog. AF. segmentation ; G, overgrowth 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. ccel. blasto- 

 co?le ; blp. blp' . blastopore ; br. 1. br.2. gills ; br. cl. branchial arches ; e. eye ; ect. ectoderm ; end. 

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

 medullary fold ; md. gr. medullary groove ; mes. mesoderm ; ma. megameres ; mi. micro- 

 meres ; nch. notochord ; n. e. c. neurenteric canal ; pcdm. proctodaeum ; pty. pituitary 

 invagination ; ret. commencement of rectum ; sk. sucker ; sp. cd. spinal cord ; st.dm. stomo- 

 daeum ; t. tail ; yk. yolk-cells ; yk. pi. yolk lug. (A D, F H, and J from Ziegler's 

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



