278 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



Fig. 7. Transverse section, shewing the secondary 

 epiblast of the cephalic plates (cp.) gradually 

 overgrown by the primary [prep.). The pro- 

 boscidial plate (prp.) arises antero-dorsally by 

 delamination. 



Fig. 8. Transverse section of slightly later stage, shew- 

 ing the two invaginations from the primary 

 epiblast on either side of the blastopore, which 

 will eventually give rise to the cerebral organs 

 (corg.). 



Fig. 9. Longitudinal section through a stage slightly 

 younger than 7. The archenteron is sub- 

 divided into intestine (int.) and oesophagus, 

 which do not communicate. 



Fig. 10. Horizontal section of older embryo. Proboscis 

 now invaginated and mesoblast accumulating. 

 The secondary epiblast, consisting of proboscis, 

 cephalic (cp.) and ventral plates (vp.), now 

 forms the external surface of the worm, having 

 sunk in, and become separated from the 

 primary epiblast (prep.). The section corre- 

 sponds to the stage shewn in figure 5. 



Fig. 11. Median longitudinal section through somewhat 

 later stage. The hinder portion of the pro- 

 boscidian mesoblast is now attached. The 

 oesophagus and intestine communicate. Rhyn- 

 chocoelom now apparent (rhc). 



