37 



Fig. 7. 'rransverse section, shewing' the secoudaiy 

 epiblast of the cephalic plates [r/j.) g'ladiialh- 

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

 boscidial plate {pTp.) arises antero-dorsally by 

 delaniination. 



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 eventiially give rise to the cerebral organs 

 [corg.). 



Fig. y. Longitudinal section through a stage slighlly 

 younger than 7. The archenteron is sub- 

 divided into intestine (int.) and oesophagus, 

 which do not communicate. 



Fi<2:. 10. Horizontal section of older embryo. Proboscis 

 now invaghiated and mesoblast accumulating. 

 The secondary epiblast, consisting of proboscis, 

 cephalic [pp-) and ventral plates {vp.), now 

 forms the external surface of the worm, having 

 sunk in, and become separated from the 

 primary epiblast {p^'ep-}- The section corre- 

 sponds to the stage shewn m figure 5. 



I'lg. 11. Median longitudinal section through somewhat 

 later stage. The hinder portion of the pro- 

 boscidian mesoblast is now attached. The 

 oesophagus and intestine communicate. Rh}n- 

 chocoelom now apparent [rhc). 



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