112 TEXTBOOK OF ZOOLOGY 



hermaphroditic or monoecious. Most of the types of animals in the 

 phylogenetic scale, up to and including the worms, are normally 

 hermaphroditic. 



Infrequent examples of hermaphrodites occur either normally or 

 occasionally abnormally here and there among the higher groups of 

 metazoans, even in man. 



In higher forms the usual method of reproduction involves germ 

 cells produced by two individuals. Each cell is either male or female, 

 the gonads of the other sex having degenerated in that individual. 

 The sexes are separate under such conditions and are said to be 

 dioecious. 



There are some forms, particularly insects, in which it is possible 

 for the unfertilized q^§ cell to develop without union with another 

 germ cell. This is known as parthenogenesis. The case of the ordi- 

 nary aphids or plant lice, known to every gardener, is a good ex- 

 ample. In the spring an egg which was fertilized and laid the pre- 

 vious fall hatches to produce an individual known as a stem-mother. 

 This individual feeds on the sap of the particular plant on which she 

 lives and grows to maturity. Instead of mating (there are no males 

 in her generation) she produces a series of eggs (macrogametes) 

 which continue to develop without union with a sperm (male germ 

 cell). Another generation of female aphids arises from these eggs 

 which in turn reproduce in a similar manner. A series of female 

 generations appears in succession during the summer. No males are 

 produced until the last generation of the season, and this time 

 there are both males and females. These mate, the females lay fer- 

 tilized eggs which pass through the winter and hatch as the first 

 generation next spring. These individuals are the stem-mothers for 

 the new season. Some authors speak of this process as "virgin 

 birth." The honey bee queen can control her offspring to some 

 degree. If her eggs are not fertilized, the offspring are all males 

 (drones). If the eggs are fertilized, as most of them are, only 

 females are produced, these becoming queens if fed abundantly on 

 proper food or workers if fed otherwise. In regard to this state of 

 affairs Lane puts it this way, "So it comes about, that though a 

 drone bee may become the father of thousands of daughters, he 

 never has a son, nor did he himself have a father." 



The eggs of a number of animals, such as frogs, molluscs, worms, 

 sea urchins, and others have been artificially stimulated to continue 



