MUSEUM OF COMPARATIVE. ZOOLOGY. 259 
to an axis passing through float and distal rim of the elevation, and 
that opposite the left, for reasons which will soon appear. 
The larva is now a little over two days old. The many cleavage 
planes, c/. p/., forming the polygonal segmentation spheres in the yolk are 
clearly defined. The protoplasmic network, 2. c., throughout the ovum, 
is likewise still well marked. The outer of the two layers, or the epi- 
blast, is ciliated externally. At the elevation on the germinative pole it 
has a reddish color. The layer beneath the epiblast, or the hypoblast, 
is thinner than the more superficial. A horizontal diameter of the egg 
is .45mm.; the longest axis at right angles to it, and passing through 
the germinative pole, is.55mm. Both epiblast and hypoblast together 
at the thickest point are not more than .01 mm. in thickness. 
Tn a slightly older larva (Pl. III. fig. 5) the significance of the notch 
at the distal rim of the primitive elevation of the germinative area be- 
comes more apparent. The epiblast and hypoblast, formerly of about 
the same thickness, have in this stage somewhat changed their relative 
dimensions, and when seen in profile are observed to have assumed folds 
which are of significance in the shape of the future covering-scale. The 
epiblast on the distal side of the disk-like elevation has thickened, and 
two well-marked angles appear on its exterior. Its surface on the distal 
side rises by a smaller angle from the yolk surface than on the proximal, 
and slopes away more gradually to the opposite side. The hypoblast 
hugs the yolk cells at all points except at one place (c.p./.), where it 
rises from them, leaving a recess which is later the cavity of the primi- 
tive larva. Near by this cavity the hypoblast is slightly separated 
from its enveloping layer, the epiblast, by a middle or third layer. The 
two angles found on the surface of the epiblast at the distal rim of the 
forming disk have grown more prominent, as shown in the two following 
sketches (figs. 6, 7), and the two layers have separated more and more 
from each other. 
When looking at the egg in its present stage of development, we no- 
tice at once how sharp the difference is between the proximal and distal 
portions of the rim of the disk-like elevation. They differ very much in 
shape from each other ; and this difference is magnified as we follow the 
course of the development into older larve. In the light of what is 
known of the existence of bilateral symmetry in the adult Agalma, we 
may regard this difference in the two borders of the scale as among the 
earliest expressions of that condition. The forming disk possesses a 
proximal and distal border, and therefore a right and left side, as referred 
to a line passing through these regions. This line lies in the same plane 
