320 MEAD. 



EXPLANATION OF PLATE XVL 



Clytnenella. 



Fig. 77. Left side : later than last figures; first division of secondary trocho- 

 blasts. 



Fig. 78. Lower hemisphere : about the 64-cell stage. 



Fig. 79. Dorsal view : about the same stage. 



Fig. 80. Left side : after 64-cell stage ; formation of secondary prototrochal 

 cells ap^, ap^, ap'' . 



Fig. 81. Right side : same egg as Fig. 77. 



Fig. 82. Lower hemisphere : same stage ; mesoderm ready to di\'ide. 



Fig. 83. Same view, a little later. 



Fig. 84. Lower hemisphere, later : last surface division of entoderm-plate 

 cells — mesoderm divided. 



Fig. 85. Same view, later. 



Fig. 86. Same view : mesoderm cells getting ready to divide again ; vacuoles 

 in three entoderm cells, A=,, B^, and C5, indicated by faint lines; also the outlines 

 of X^ and X^ ; entoderm plate complete ; prototroch complete. 



Fig. 87. Same view, little later stage. 



Fig. 88. Lower hemisphere, somewhat dorsal : mesoderm cell divided; m and 

 m correspond to the very minute cells in Amphitrite (of. Fig. 50, m and m). 



