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 definiteness 
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 which 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%? 5°4, 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 
