THE FROG. 167 



e. At 4 — 5 days (typical blastopore stage. Cf. i. /). 

 Longitudinal vertical, to pass through the blasto- 

 pore. 



a. The lower layer cells ; now practically destitute 

 of pigment and almost completely enclosed in 

 epiblast; exposed only in the region of the 

 blastopore. 



/i. The archenteron (primitive alimentary canal); a 

 spacious cavity occupying much the position of 

 the cleavage cavity in Section d) in communi- 

 cation with the exterior at the blastopore. 



y. The cleavage cavity ; small and irregular, situated 

 below yS at the end opposite the blastopore. 



& The hypoblast (inner germinal layer or digestive 

 epithelium) ; a single layer of closely packed 

 cells forming the roof of the archenteron, con- 

 tinuous with the epiblast at the upper lip of 

 the blastopore (i. e. formed as the result of in- 

 vagination of the latter layer). 



€. The mesoblast (middle germinal layer) ; obvious 

 in median longitudinal section as two series of 

 cells somewhat irregular and more loosely scat- 

 tered than the rest ; an upper series lying in the 

 interspace between the roof of the archenteron 

 and adjacent epiblast ; a lower series somewhat 

 similarly interposed below between the lower 

 layer cells and adjacent epiblast, and most 

 numerous at the blastopore. 



f. The same at 12 — 14 days (cf. i. Ji). Transverse 

 across the middle of the neural plate. Examine 

 under a high power. 



