62 ESSENTIALS OF ZOOLOGY 



repeated divisions. In a number of the Sporozoa, both sexual and 

 asexual generations occur. The zygotes, which are formed in the 

 sexual phase or generation, produce a number of spores which de- 

 velop sporozoites (already studied under Plasmodium). These be- 

 come nutritive trophozoites and are capable of production of another 

 generation of gametes. Conjugation of Paramecium is also looked 

 upon as a forerunner of sexual reproduction. 



In simple Metazoa there are likewise two forms of reproduction: 

 asexual (without sex), including budding and fission, and sexual, 

 which involves the union of two germ (sex) cells, one male and one 

 female. In simple forms like sponges and jellyfish the germ cells 

 arise from general formative interstitial cells between the two primi- 

 tive germ layers to form temporary gonads. When the germ cells 

 are mature, they break through the wall to the outside of the body. 

 Again, among the simpler metazoans a single individual produces 

 both male and female germ cells. Such an organism is said to be 

 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 egg 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 



