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MANUAL OF ELEMENTARY ZOOLOGY 



by the opercula they are not seen till a later stage. About 

 the end of the second month the lungs which have been 

 forming come into use and the gills start to degenerate, 



so.m 

 ap.m. 



Fig. "374. — Sections of an embryo frog at about the stage of Fig. 373. 

 Ay Transverse ; B t longitudinal. 



an., Anus cml., ccelom ; ect., ectoderm or epiblast ; end., endoderm or hypoblast ; 

 f.br., fore-brain; h.br. t hind-brain; ht. t rudiment of heart; int., intestine; 



l.p.y lateral plate of mesoblast ; lr., rudiment of liver ; m.br. % mid-brain ; m.s., 

 [*j, mesoblastic somite ; mes. f mesoblast ; tries',, mesoblast continuous with epiblast 

 auKL of neural canal and hypoblast of notochord in region of primitive streak ; ne.c, 

 - .- neurenteric canal (closing); nch. y notochord; p/i.^ pharynx); /»/,, rudiment of 



pituitary body ; so.m., somatic mesoblast ; j/.c, spinal cord ; .?/./«., splanchnic 



mesoblast ; stm. t stomodaeum. 



and a fortnight later the tadpole begins to turn into a young 

 frog. The outer layer of the skin and the horny jaws are 

 thrown off, the mouth enlarges and changes its shape, the 

 fore-limbs appear, that on the left being pushed through 

 the gill opening, that on the right breaking through the 



