Development and Metamorphosis 
37 
either the germinal or the food part of the eggs. There seems to exist no 
differentiation among these cells at first, but soon certain ones begin to 
move slowly down through the egg-tube in single file, each becoming sur¬ 
rounded and enclosed by yolk, i.e., reserve foodstuff. This gathering of 
yolk increases the size of the forming eggs, so that they appear as a short 
string of beads of varying size enclosed in the elastic egg-tube. When of 
considerable size each egg in the lower end of the tube becomes enclosed 
Fig. 67.—Insect eggs and parts of eggs, showing micropyle. a, egg of Drosophila cel- 
laris; b, upper pole of egg of robber-fly, Asilus crabriformis; c, upper pole of egg 
of hawk-moth, Sphinx populi; d, egg of head-louse, Pediculus capitis; e, egg of 
dragon-fly, Libellula depressa; f, upper surface of egg of harpy-moth, Harpyia 
vinula; g, upper pole of egg of Hammalicherus cerdo; h, upper pole of egg of sul¬ 
phur-butterfly, Colias hyale. (After Leuckart; much enlarged.) 
in two envelopes, a membranous inner one (yolk or vitelline membrane) and 
an outer horny one, the chorion or egg-shell. But both of these envelopes 
are pierced at one pole by a tiny opening, the micropyle (Fig. 67), and 
through this opening the fertilizing spermatozoa enter the egg from the 
seminal receptacle just before the egg is extruded from the body. 
The development of the embryo within the egg is also securely sealed 
away from the eyes of most amateurs. The study of insect embryology 
requires a knowledge of microscopic technic, and facilities for fixing and 
