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 



ec ent 



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

 bl.casl, blastoccel; mi, micromeres; mg, macromeres. B, formation of medul- 

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

 of egg in stage B to show germ-layers, bl.ccel, blastoccel ; blp, 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, stomodseum; t, 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 invagination soon appears just above the 

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

 into the mouth. 



The invagination (proctod&um, 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 



