l6o ELEMENTARY BIOLOGY. [CHAP. 



the whole having undergone a slight increase in 

 total capacity and lost its spherical shape. 



a. The blastopore ; still visible but small and 

 cxcentric (displaced towards the flattened or 

 dorsal surface). 



ft. The nciwal plate ; obvious as a lyre-shaped flat- 

 tening, in front of and in a line with the blasto- 

 pore ; its edges thickened and raised up {neural 

 folds), its mid region depressed {jie it rat groove). 



The neural folds will in all probability be seen 

 to unite in front, and die away behind, at the sides 

 of the blastopore. A more or less marked ap- 

 proximation of their hinder halves will be obvious. 



/'. llie same at 12 — 14 days. 



The embryo may now be definitely recognized as 

 a pear-shaped body still enclosed within the mu- 

 cus investment. Remove and examine it. 



a. The body ; head and trunk, very obvious if seen 

 from the dorsal aspect. 



/?. The neural folds ; seen, if examined from the 

 dorsal aspect, to be uniting in the middle line. 



y. The blastopore ; recognizable as a minute pore 

 behind and below ft. 



Z. The suckers ; two oval thick-lipped depressions 

 on the under side of the head (mouth not yet 

 recognizable). 



€. The visceral arches ; generally to be seen at this 

 stage, as a couple of oblique ridges on cither 

 side, above S. 



/. T/ie e/nbryo at 15 — 18 days. 



Still enclosed within the mucus mass ; recogniz- 



