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, Asiliis crabrijormis; c, upper pole of egg 

 of hawk-moth, Sphinx popiili: d, egg of head-louse, Pedkiilus capitis; e, egg of 

 dragon-fly, Libellula depressa; /, upper surface of egg of harpy-moth, Harpyia 

 vinuhi; 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 facihties for fixing and 



