178 ALICE JOHNSON. 



Fig. 12. — Longitudinal section through the bind part of an embryo with 

 about twelve somites. 



Pig. 13. — Longitudinal section through an embryo with about eighteen 

 somites. 



Fig. 14. — Transverse section through an embryo before the formation of 

 the tail, showing the open blastopore. 



Figs. 15, 16, 17. — Transverse sections through a considerably older embryo 

 than that of fig. 14, showing continuity of blastopore with hind.gut. Fig. 15 

 is the most posterior, and fig. 17 the most anterior of the series. 



The liglit grey colour signifies the epiblast and organs derived from it, the 

 dark grey signifies mesoblast, and the yellow signifies hypoblast and organa 

 derived from it. 



