1958 ] Woodland — Thermobia domestica (Packard) 13 
The external appearance of the newly laid egg of the 
firebrat has been described by Adams (1933), Sweetman 
(1938), and Remington (1948). The present study reveals 
that the endochorion (Fig. 5, EN) is about 2 microns thick. 
Its thickness does not vary appreciably over the whole egg. 
There adheres to its outer surface a single layer of spherical 
droplets, each about 11 microns in diameter immediately 
after oviposition. The droplets are rather irregularly placed, 
but tend to be in groups with large intervening spaces. In 
the spaces are droplets many times smaller than the large 
ones. Within a few hours a change occurs. The large 
droplets break up into rather evenly spaced ones about 3 
microns in diameter (Fig. 5, CD). Usually a space of 2 
microns now occurs between droplets, although pairs of 
contiguous droplets are sometimes seen. The very minute 
droplets are still scattered among the larger ones. The 
appearance of the droplets is essentially the same on shed 
chorions long after the nymphs have left them. Possibly 
the droplets are present as a thin continuous film before 
oviposition. 
The droplets adhere firmly to the exochorion. A few 
individual droplets are separated from it by sectioning, but 
the other procedures to which the eggs have been subjected 
practically never dislodge them. In contrast, the exochorion 
is loosened from the endochorion when the egg contents 
shrink during fixation, owing to dissolution of the fatty 
elements. Moreover, the exochorion is normally loosened 
from the endochorion during development, as the volume 
of the egg contents is diminished slightly. The surface of 
the endochorion is then seen to be reticulated into hundreds 
of small polygonal areas. Most of these polygons are fairly 
regular hexagons with diagonals usually between 55 and 85 
microns long. The wall separating adjacent areas is 5 
microns thick. 
Sweetman (1938) supposed this hexagonal reticulation 
to be produced by the cells of the embryonic tissues, but 
the present study showed it to be produced by the follicular 
cells that secreted the endochorion. The large size of the 
hexagons emphasizes the amount the egg has grown since 
the endochorion was secreted. 
