BIGELOW: EARLY DEVELOPMENT OF LEPAS. 105 
clusively that there are, besides the four ‘secondary mesoblasts,” two 
entoblasts and two dividing primary mesoblasts in the egg of this stage. 
The cells of the anterior pair of ‘‘ secondary mesoblasts ” (67°, 0'7) are 
always hemispherical in form (Fig. 73), while those of the posterior pair 
are flattened between the primary mesoblast cells (d**, d®*) and the 
blastoderm (Fig. 72). It also appears from the figures that the two 
derivatives of the primary mesoblast (d*”), the two pairs of ‘ secondary 
mesoblasts,” and the two entoblasts, are arranged according to a plan 
of bilateral symmetry. The division plane in the yolk (Fig. 73) is the 
cleavage plane formed between the entoblast cells during the fifth cleav- 
age. With this brief description of the sixty-two-cell stage we may now 
turn to a more detailed consideration of the sixth cleavage, which formed 
the stage. 
The large number of small cells and the absence of “landmarks” 
makes rapid and certain identification of individual cells of the blasto- 
derm on the dorsal surface impossible in the sixty-two-cell and later 
stages. By carefully comparing drawings of stages in which the cells 
of the blastoderm are in early and late stages of mitosis, it is often 
possible to identify all the individual blastoderm cells in the sixty-two- 
cell stage. But since it is impossible to follow the blastoderm cells 
to their fate in organs of the Nauplius, I have not attempted to give in 
this account the lineage of all cells after the thirty-two-cell stage. 
After that stage the most important cells concerned with the germ- 
layers are near the blastopore. These are followed easily and with 
certainty. 
During the fourth and fifth cleavages the blastoderm was greatly 
extended by the flattening of its cells and by the increase of surface 
associated with cell-division. This is repeated during the sixth cleavage, 
and the result is that the blastoderm in the majority of cases is com- 
pleted, the yolk-entoblast cells being no longer exposed to the exterior 
at the blastopore (see Plate 7, Fig. 56, and Plate 8, Fig. 71). 
In most cases a very small opening between the blastoderm cells 
represents the remnant of the blastopore. In fact the cells bounding 
the blastopore rarely come so closely together in this stage as to com- 
pletely obliterate the opening (see Plate 7, Figs. 57, 60, 62; Plate 8, 
Fig. 71; Plate 2, Fig. 76). This persistence of the blastopore has 
been of great service in determining the origin of the “‘ secondary mes- 
oblasts” and in the orientation of succeeding stages. 
Along with the growth of the blastoderm over the blastopore during 
