BIGELOW: EARLY DEVELOPMENT OF LEPAS. 103 



About the time that the fourteen blastoderm cells have completed 

 their division, the primary mesoblast cell (d^-'^) prepares to divide, its 

 spindle being transverse to the long axis of the egg (Plate 5, Fig. 48). 

 The cleavage plane coincides with the sagittal plane of the embryo, and 

 the resulting cells form the posterior boundary of the blastopore (Fig. 

 52). The constant and definite position of these two mesoblast cells, 

 their retarded division, which gives them distinctive nuclear phases, 

 their tendency to stain less intensely than other cells, the definiteuess 

 of the position and cleavage direction of the surrounding cells — all 

 these features make it possible to identify positively the derivatives of 

 the primary mesoblast cell (d^-^) in this and the following stages. 



The yolk-cell (entoblast, d^-^) is the last cell to undergo the fifth 

 cleavage ; it commonly divides about the time that the blastoderm cells 

 prepare for the next (sixth) cleavage ; but at times the cleavage of the 

 entoblast is so delayed as to be nearly simultaneous with the sixth 

 cleavage of the blastoderm cells. The nuclear spindle is usually almost 

 perpendicular to the sagittal plane (Figs. 52, 116, 117). A cleavage 

 plane, dividing the yolk nearly equally makes its appearance at this 

 stage, but it becomes more clearly visible about the time that the next 

 division takes place in the blastoderm cells, and it may therefore be 

 described later, in connection with the figures w:hich illustrate the 

 account of the sixth cleavage. 



The blastoderm has been greatly extended since the last stage., owing 

 to the multiplication of its cells by division, and to the accompanying 

 increase of surface produced by the flattening of the cells. The blasto- 

 pore has become less extensive as the yolk-cell (entoblast) has become 

 more completely covered (Plate 6, Figs. 51, 54 ; Plate 8, Fig. 69). It 

 is filled by the protoplasmic portion of the yolk-entoblast, and is bounded 

 posteriorly by the two primary mesoblast cells {d^'^, d^-*), anteriorly 

 and laterally by the four mes-ectoblast cells (a'•^ 6®'^ 6®*, c^-^). With 

 the exception of these four cells, which are in contact with the yolk- 

 entoblast at the blastopore, all other cells of the blastoderm are purely 

 ectoblastic. 



Figures 47-55 (Plate 6), 69, 70 (Plate 8), and 114-117 (Plate 12), 

 show the details of cell arrangement in the thirty-two-cell stage. There 

 is slight variability in the adjustment of the cells to one another, but 

 examination of the figures shows that the relative positions of the cells 

 are the same in all cases. In good transparent preparations I have 

 seen hundreds of eggs in the thirty-two-cell stage conforming to the 

 conditions shown in the figures, very few in which the arrangement of 



