BIGELOW. Development of Lepas. 



PLATE 6. 



Figures from transparent preparations of entire eggs. Vegetative pole at the 

 right in figures representing lateral views. 



Fig. 47. Sixteen-cell stage with all cells of the blastoderm in fifth cleavage. 

 Primary mesoblast (d*- 2 ) and entoblast (d 5 ' 1 ) with enlarging nuclei. 

 Lateral view. 



Figs. 48 and 61. Eggs with thirty cells, but the primary mesoblast cell (c? 5 * 2 ) has 

 not yet completed the fifth cleavage. Nucleus of entoblast cell (<f 5>1 ) 

 still in " resting " phase, but chromosomes preparing for fifth cleavage. 

 Entoblast (blastopore) bounded anteriorly and laterally by mes-ecto- 

 blasts (a 6 ' 3 , 6 G>3 , 6 54 , c 6 ' 3 ). Viewed from vegetative pole. 



Figs. 49, 50 and 53. Same stage seen in lateral view. In Figure 53 more of the 

 dorsal than of the ventral side is seen. Comparison shows that the cells 

 have essentially the same positions in the three eggs. 



Fig. 52. Egg with thirty-two cells, reckoning the dividing yolk-entoblast as two 

 cells. Derivatives (rf 6 ' 3 , c? 6>4 ) of the primary mesoblast at the posterior 

 edge of entoblast (blastopore). Viewed from vegetative pole. 



Fig. 54. Optical section in sagittal plane of egg similar to one represented in 

 Figure 50. Cleavage cavity occupied by the yolk-entoblast, which is 

 uncovered at the blastopore only. 



Fig. 55. View from animal pole of egg represented in lateral view in Figure 53. 



