106 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the sixth cleavage, the two primary mesoblast cells (d*°, d®°*) are 
crowded into the yolk beneath the blastoderm, pushing the two en- 
toblast nuclei deeper into the yolk (Plate 7, Fig. 59). The primary 
mesoblast cells thus come to lie beneath the blastoderm at the posterior 
end of the embryo. As in the two preceding stages, they are easily 
identified by their distinguishing features, and furthermore the divisions 
of all surrounding cells are accounted for, so that there can be no doubt 
of the lineage of the primary mesoblast cells. In series of eggs in 
various phases of the sixth cleavage the primary mesoblast cells have 
been seen in their successive positions, from that of the thirty-two-cell 
stage to that of the sixty-two-cell stage. At a time when some ecto- 
blastic cells are undivided and the blastoderm is not completed, the 
two primary mesoblast cells are seen filling the blastopore and in part 
exposed to the exterior, but as the blastopore becomes closed they sink 
into the yolk, and the blastoderm closes over them. 
The primary mesoblast cells (d**, d%*), before the sixth cleavage 
takes place in them, may be symmetrically placed with reference to 
the sagittal plane (Plate 7, Fig. 64; Plate 8, Fig. 72; Plate 12, 
Fig. 120); but more often one (d°*) is found in a position dorsal 
or anterior to the other (Figs. 56, 59, 60, 71). In the majority of 
eggs the two cells appear to have undergone torsion as the blasto- 
derm closed around and over them. In the thirty-two-cell stage 
they are usually symmetrically placed side by side, but even in 
this stage there may be some shifting, as shown in Figure 52 (Plate 
6). Figures 62 and 63 (Plate 7) show a very common condition, 
in which they have been so turned that the cleavage plane between 
them no longer coincides with the sagittal plane. In all such cases 
they appear to retain their original positions with reference to the 
right and left sides of the embryo. The various positions occupied 
by these cells may be the result of shiftings in adjustment to least 
resistance at the time when the overgrowing blastoderm crowds them 
inwards. 
The spindles concerned with the sixth cleavage of the two derivatives 
(d®-8, d°*) of the primary mesoblast cell are more often about perpen- 
dicular to the long axis of the egg (Plate 7, Fig. 56), but sometimes 
almost parallel to that axis; all intermediate conditions are seen. In 
Figures 65 (Plate 7) and 121 (Plate 12) the two cells are represented 
as having completed the sixth cleavage, so that there exists a stage 
with sixty-two cells. Immediately after division the four resulting 
cells (d 7°) are rounded, as shown in Figure 65, but soon afterwards 
