I.] 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. 



/3. 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 ft at the end opposite the blastopore. 



8. 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). 



c. 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. ti). Transverse 

 across the middle of the neural plate. Examine 

 under a high power. 



