314 MEAD. 



EXPLANATION OF PLATE XIIL 



Amphitrite. 



Prototroch, brown ; somatic plate, blue ; mesoderm, red ; entoderm, stippled ; 

 the groups of somatic-plate cells descended from X^, x^, and x^ separated by 

 heavy lines. 



Fig. 41. Lower hemisphere : mesoderm cells beginning to sink below the 

 surface. 



Fig. 42. Same view : mesoderm cells M and M partly covered and ready to 

 divide and give rise to the minute cells seen in Fig. 46, m and m. 



Fig. 43. Similar view, somewhat to left side. 



Fig. 44. Same view as last figure and about the same age. The two 

 figures show the variations in time of division. 



Fig. 45. Dorsal view, showing the dorsal interruption in the prototroch. 



Fig. 46. Lower hemisphere, dorsal : cells of entoderm plate dividing for 

 the last time on the surface ; ends of mesoderm cells still seen at the surface ; 

 m and m, small cells in segmentation cavity arising by division of the original 

 mesoderm cells M and M (cf. Figs. 42 and 44). 



Fig. 47. Lower hemisphere : definitive entoderm plate ; mesoderm entirely 

 within the segmentation cavity. 



Fig. 48. Dorsal view : division of x^. 



Fig. 49. Lower hemisphere, dorsal. 



Fig. 50. Lower hemisphere, dorsal : mesoderm cells below the surface ; the 

 mesodermal teloblasts ready to divide the second time. 



Fig. 51. Dorsal view, showing interruption of prototroch. 



Fig. 52. Lower hemisphere : each mesoblast band composed of three cells. 



