Plate II. 



Fig. 21. Furmatioii of bilateral cells. Transverse section of a stii^e sliglitl}' 



more advanced than in Fig. 20. 

 Fig. 22. Formation of eighth ectomere-qnartet. Horizontal ^-ection thnnigh 



a[)ical pole. 

 Fig. 23. »Same stage. Transverse section. 

 Fi<i'. 24. Stajie before cleavage of mesomere-cell ; a)nstrncted from .M'clions 



so as to show the mutual relative position of cells. Cells furming 



the ventral side of the egg are colored blue. 

 Fig. 25. Cleavage of mesomere-. and entoraere-cells. Horizontal section 



through vegetative pole. 

 Fig. 2G. Cleavage of mesomere-cell M. Transverse section. 

 Fig. 27. Completion of same stage, showing the relative [)osition of ecto- 



mere-quartets. Horizontal section. 

 Fig. 28. Formation of tirst mesoderm-cells. Sagittal section. 

 Fig. 29. Same stage. Transverse section. 

 Fig. 30. Stage after formation of tenth ectomere-qnartet. Horizontal section 



near vegetative pole. 

 Fig. 31. Horizontal section through vegetative pole, showing complete en- 

 closure of mesomere-cells l^y epibolic growth of entoderm-cells. 

 Fig. 32. Cleavage of second mesoderm-cells. Horizontal section. 

 Fig. 33. Same stage. Sagittal section. 



Fig. 34. Formation of second entoderm-cells. Horizontal section. 

 Fig. 35. Cleavage of third mesoderm-cells. Transverse section of the [»osterior 



side. 

 Fig. 36. Formation of fourth mesoderm-cells. Sagittal section. 

 J''ig. 37. Eggs of Aplysia. Fresh. 

 Fig. 38. Same, highly magniüed. Fresh. 



Fig. 39. Conuuencement of tirst cleavage stage. Side view. Fresh. 

 Fig. 40. Formation of blastomere -cells. Side view. 



Fig. 41. A slightly more advanced stage of Fig. 40. A}»ical vi(;w. Fresh. 

 Fig. 42. Com[)letion of same stage, showing the pro[)er positions of blasto- 



meres. Apical view. Fresh. 



