30 INSECT TRANSFORMATION 



starfish or a mouse for example, it is enormous, though even 

 such small eggs as those are very large for single cells. 



The rich store of food-material in the yolk of an insect's 

 egg is an important factor with regard to the creature's life- 

 history because it ensures that development will have advanced 

 to a considerable extent before the crisis of hatching shall 

 usher the young insect into the surroundings of the outer world. 

 The egg is protected by a firm horny case or shell, which has a 

 characteristic shape and appearance for insects of different 

 groups (Fig. 1 6). Eggs arise by a series of divisions from the 

 primitive germ-cells, and are at first minute bodies consisting 

 mainly of living protoplasm each with its distinct internal 

 nucleus of highly complex structure the centre of the cell's 

 activities. As growth goes on, yolk accumulates in the cell 

 which, when ripe, is found to have its external region within the 

 envelope mainly protoplasmic while the central mass consists 

 chiefly of inert food-material. 



Most insect eggs as is the case among the vast majority 

 of animals require to be fertilized before they begin to develop. 

 The essential process of fertilization is the entrance into the 

 egg of the head of a spermatozoon or active, vibratile sperm- 

 cell, large numbers of which are received into the female's 

 spermatheca when she pairs with a male. The head of the 

 spermatozoon consists chiefly of the sperm-nucleus, which unit- 

 ing with the egg-nucleus forms the zygote-nucleus whence all 

 the nuclei of the myriad cells that build up the body of the 

 offspring are derived by repeated series of divisions. There 

 are, however, a large number of cases now known among insects 

 in which the egg is capable of development without fertiliza- 

 tion ; these afford examples of parthenogenesis or virgin-repro- 

 duction. The numerous successive generations of aphids or 

 " greenfly " during spring and summer, which are all females, 

 and the " drones " of the hive-bee and many related insects, 

 which are males, arise from parthenogenetic eggs. Many 

 highly interesting problems of inheritance have been elucidated 

 by recent researches on the details of nuclear behaviour during 

 the maturation of the germ-cells and the fertilization of the 

 eggs in various insects. 1 But without pausing to discuss 



1 L. Doncaster : " An Introduction to the Study of Cytology". Cam- 

 bridge, 1920. 



