BIGELOW: EARLY DEVELOPMENT OF LEPAS. 109 
the cell arrangement and spindles as shown in Figure 58, 60 and 62 
gives evidence entirely in favor of the explanation given of the cell- 
lineage of the “secondary mesoblasts.” They are certainly derived 
from the ectoblastic cells of the blastoderm, and the evidence com- 
pletely supports the interpretation that they are derived directly from 
the cells bounding the blastopore laterally and anteriorly in the thirty- 
two-cell stage. 
The cell-lineage of the “ secondary mesoblasts ” is, then, as shown in 
the following table (see also complete table of the cell-lineage on page 
135). 
aii 7 right anterior ‘secondary mesoblast” cell. 
6:3 
b7°6 blastoderm cell (ectoblast). 
b7-7 left anterior ‘“‘secondary mesoblast” cell. 
p64 ii %h 
ie b7-8 blastoderm cell (ectoblast). 
a’ left posterior “secondary mesoblast” cell? 
ot 
a’6 blastoderm cell (ectoblast). 
ct5 right posterior “secondary mesoblast ” cell. 
63 <ci 
c’6 blastoderm cell (ectoblast). 
It will be noticed that “secondary mesoblast” originates from the 
quadrants a, 6, and c. One cell each is contributed by a and e but two 
cells come from 6, Tracing the lineage to the three micromeres which 
are separated from the yolk-macromere in the first three cleavages, it is 
found that only the first (ad?) and the second (c*) contain “ secondary 
mesoblast ” ; the third (d*) is purely ectoblastic. 
After the sixty-two-cell stage the derivatives of the “ secondary meso- 
blasts”’ have not been distinguished from those cells which were derived 
from the primary mesoblast. The cells of the two origins become 
mingled together and there appear to be in Lepas no distinguishing 
characteristics. Hereafter the term mesoblast (ms’b/. in the figures) will 
be used in the description as including the mesoblast cells of the two 
origins, 
The entoblast nuclei (d°’, *?) are always near the primary mesoblast 
cells, but, as shown in the figures, they occupy no constant position in 
relation to particular cells. They stain more intensely than the nuclei 
