l6o ELEMENTARY BIOLOGY. [CHAP. 



the whole having undergone a slight increase in 

 total capacity and lost its -spherical shape, 

 a. The blast opor e ; still visible but small and 



excentric (displaced towards the flattened or 



dorsal surface). 



y8. The neural 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 (neural 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. 



h. The same at 12 14 days. 



The embryo may now be definitely recognized as 

 a pear-shaped body still enclosed within the mu- 

 cous 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 ; no longer recognizable. 

 S. The suckers ; two oval thick-lipped depressions 

 on the under side of the head (mouth not yet 

 recognizable). 



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

 stage, as a couple of oblique ridges on either 

 side, above 8. 



/. The embryo at 15 18 days. 



Still enclosed within the mucus mass : recogniz- 



