1900] LIFE HISTORY OF SILPHIUM 119g 
figures are of sections. ig. 76 shows one half of an embryo in 
which all four upper octants have divided. Of the two shown 
in the figure, the left one has cut off the dermatogen by a peri- 
clinal wall, while the right one shows the usual anticlinal wall. The 
other two of the’ four show in the adjacent section of the series, 
each with an anticlinal wall, as in fig. 77, which is drawn from 
another embryo in nearly the same stage. These anticlinals may 
intersect either of the primary, or octant, walls, and sometimes, 
by their early shifting, one may be in doubt as to which wall 
really separates the octants (¢. g., fig. 78). 
The dermatogen is usually cut off by the second series of 
walls in each upper octant (figs. 78, 8r-right, 82-left). In 
octants where the first wall was periclinal, differentiating the 
dermatogen, the second wall is usually an anticlinal, in the inner 
of the two cells by the first division ( fig. 8r-left). The next 
walls generally occur as a series of periclinals, giving three con- 
centric layers of cells in the upper octants, as in fig. 53. The 
regularity in this part of the embryo is soon lost, and there arises 
a mass of parenchymatous tissue in which division may occur in 
any direction. By repeated anticlinal divisions the dermatogen 
grows to keep pace with the increase in the bulk of the tissue 
beneath it. 
When each upper octant has undergone its first division, the 
four lower octants are still undivided (jigs. 76, vi \, bine der- 
matogen is cut off here by the first walls ( figs. 78, 70), Lhe 
second division is probably always in the inner cell, thus sepa- 
rating periblem from plerome. Fig. 79 (left) is not conclusive 
on this point, but fig. 84, in which a series of transverse walls 
has arisen, shows at the left that periblem and plerome cells 
arise by the division of an original inner cell. Asa result of 
these divisions three concentric layers of cells are formed in the 
oe es part of the embryo, as is shown in jigs. 80, 81. 
While the primary tissues are thus being differentiated, the 
cells of the lower octants are becoming considerably elongated 
( figs. 78-81); and soon a series of transverse walls appears, by 
Which two similar tiers or layers of cells are formed, as shown 
