86 The Life-History of Insects 



leaves her eggs in the prepared cells, the Ichneumon- 

 fly in the body of the living caterpillar, the Burying- 

 beetle in the dead body of a bird, the Sawfly in the 

 leaf of a plant. 



Fertilization. — It will be remembered that into 

 the genital pouch of the female cockroach opens the 

 sperm reservoir, where are stored the active sperm- 

 cells derived from the male insect. As each egg 

 passes from the vagina to its place in the egg-capsule, 

 one of its ends comes in contact with the opening of 

 the sperm reservoir. The hard coat at this end is 

 perforated with a number of minute holes, through 

 which a sperm-cell can make its way into the proto- 

 plasm of the egg. Here the nucleated head of the 

 active sperm-cell (male pronucleus) unites with the 

 nucleus of the egg-cell {female pronucleus) (fig. 57> 

 A, m.f.). The nucleus {segmentation nucleus) formed 

 by their union gives origin by repeated division (fig. 

 57 B, s.) or segmentation to the embryo. This union 

 of the male and female elements is known as the 

 fertilization of the egg, and in most insects, as in 

 animals generally, no new individual will be developed 

 if the egg remain unfertilized. 



Polar Bodies. — Before fertilization, the nucleus 

 of the egg-cell twice undergoes division, the separated 

 portions, each with a small quantity of protoplasm, 

 being cast out, as the two polar bodies or directive cells 

 (fig. 57, A, pi, pi). By the separation of these the 

 substance of the nucleus is reduced to a quarter 

 of its original amount. It will be remembered that, 

 in the male, four active sperm-cells are formed by 

 the division of each sperm mother-cell (spermatocyte) ; 

 and it is believed that when fertilization takes place, 

 the nuclear matter cast out by the formation of the 

 second polar body is replaced by an equivalent amount 

 derived from another individual — the male. The 



