1958 ] Woodland — Thermobia domestica (Packard) 23 
Tiegs (1940), however, deems it erroneous to consider 
the type of cleavage as simply the mechanical result of the 
quantity of yolk within the egg. In support of this conten- 
tion he says that the eggs of symphylans are not unusually 
small, though cleavage is total. But consider the following 
comparison. The egg of Hanseniella is spherical, averaging 
0.37 mm. in diameter; the long diameter of the ellipsoidal 
egg of Thermobia averages 1.00 mm., the short diameter 
0.80 mm. At hatching, Thermobia measures 1.5 mm. long, 
exclusive of appendages. We deduce that Hanseniella also 
measures close to 1.5 mm. long at hatching, for the follow- 
ing reason. The circumference of a sphere 0.37 mm. in 
diameter is 1.2 mm. The body of the embryo is curved in a 
circle around the entire circumference of the egg, but is a 
little longer than the circumference since the bent head is 
directed inward. We therefore conclude that the egg of 
Thermobia contains a relatively much larger percentage of 
yolk than does the egg of Hanseniella . 
Tiegs further supports his statement by noting that some 
tiny, yolkless insect eggs have superficial cleavage (Fer- 
nando, 1934). But there is no reason to suppose that super- 
ficial cleavage, once established, would not be as satisfactory 
for yolkless as for yolk-rich eggs. We do not consider, how- 
ever, that this fact invalidates the theory that superficial 
cleavage was originally developed as an adaptation to large, 
yolk-rich eggs. 
Although only one species of perlarian has been thorough- 
ly studied embryologically, Miller’s detailed account (1939, 
1940) of Pteronarcys proteus reveals a number of similari- 
ties between the embryology of this perlarian and that of 
lepismatids: (1) Practically no cytoplasm is present in 
freshly laid eggs. (2) The embryonic rudiment represents 
chiefly cephalic material. (3) The germ band is of the im- 
mersed type. (4) The inner layer represents mesoderm 
only and is produced without the formation of a ventral 
furrow. (5) The mesoderm of the eleventh abdominal seg- 
ment shows no coelomic sacs. (6) Transitory appendages 
appear on the intercalary segment and on abdominal seg- 
ments two to ten. 
