508 



COLLEGE ZOOLOGY 



along the median dorsal line, and the -embryo now lengthens in 

 this direction. The region where the yolk plug was situated 

 lies at the posterior end. On either side near the anterior end 

 two gill-arches appear (Fig. 425, D, br.cl), and in front of each 

 of these a depression arises which unites with its fellow and 



ect 



end. 



ent 



FIG. 425. Development of the embryo of the frog. A, section of blastula. 

 bl.ccel, blastocoel; mi, micromeres; mg, macromeres. B, formation of medul- 

 lary groove, md.gr, and medullary fold, md.f; yk.pl, yolk-plug. C, section 

 of egg in stage B to show germ-layers, bl.ccel, blastoccel ; Up, blastopore; 

 ect, ectoderm; end, entoderm; ent, enteron; mes, mesoderm; nch, notochord; 

 yk.pl, yolk-plug. D, older embryo, br.cl, branchial arches; stdm, stomo- 

 daeum; /, tail. E, newly hatched tadpole. br>i, br.2, gills; e, eye; pcdm, procto- 

 daeum; sk, sucker; stdm, stomodaeum; /, tail. (From Parker and Haswell; 

 A, D, after Ziegler's models; B, C, E, after Marshall.) 



moves to the ventral surface, becoming the ventral sucker (Fig. 

 425, E, sk). An in vagina tion soon appears just above the 

 ventral sucker; this is the stomodceum (stdm) which develops 

 into the mouth. 



The invagination (proctodceum, Fig. 425, E, pcdm) which 

 becomes the anus appears beneath the tail (/) at the posterior 

 end. On either side above the mouth a thickening of the 



