160 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 



excentric (displaced towards the flattened or 



dorsal surface). 



/?. 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. 

 B. 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. 



i. The embryo at 15 18 days. 



Still enclosed within the mucus mass; recogniz- 



