1 68 THE AMPHIOXUS. 



PLATE II. 



All the Figures are drawn from the living object. 



Fig. 13. Further stage with eight large lower cells and five 

 sixteen-celled spheres. On one of the latter all the cells are 

 biscuit-shaped, in the act of division. 



Fig. 14. The same embryo in optical section. 



Fig. 15. Further stage, lateral view. 



Fig. 16. The same in optical section. 



Fig. 17. Further stage, lateral view. 



Fig. 18. The same in optical section. The epiblast cells by 

 this time assume the character of an epithelium. 



Fig. 19. Further stage (Blastula), lateral view r . 



Fig. 20. The same in optical section. 



Fig. 21. The lower pole begins to flatten out ; optical section. 



Fig. 22. The invagination is in process ; optical section. 



Fig. 23. The invagination has advanced further ; optical 

 section. 



Fig. 24. Stage of the completed invagination in optical 

 longitudinal section. 



PLATE III. 



All the figures, with the exception of 35, 36, and 37, are 

 drawn from the living object. 



Fig. 25. Stage of the completed invagination (comp. Fig. 24), 

 as seen from the gastrula-mouth. 



Fig. 26. Further stage in optical longitudinal section. 



Fig. 27. The same stage turned 90 round the primary axis 

 (drawn from the upper to the lower pole) ; optical section. 



Fig. 28. The same stage, as seen from the gastrula-mouth. 



Fig. 29. Further stage in optical longitudinal section. 



