126 THE MICROSCOPIST. 



stances the sexes are distinct, and the species are called 

 dioecious. 



9. The alternation of generations is a term given in bi- 

 ology to express a form of multiplication which occurs in 

 some of the more simple forms of life. It consists really 

 of the alternation of a true sexual generation with the 

 phenomenon of budding. Thus a fern spore gives rise, by 

 budding and cell-division, to a prothallium ; this produces 

 archegonia and antheridia, as the sexual elements are 

 called, and the embryo which results from sexual union 

 produces not a prothallium but a fern. This phenomenon 

 is better seen in the Hydrozoa. In these the egg produces 

 a minute, ciliated, free-swimming body, which attaches 

 itself, becomes tapering, develops a mouth and tentacles, 

 and is known as the Hydra tuba,. This multiplies itself, 

 and produces extensive colonies by germination, but under 

 certain circumstances divides by fission and produces 

 Medusce, which develop ova. 



10. Parthenogenesis designates the production of new 

 individuals by virgin females without the intervention of 

 a male. It has also been applied to germination and 

 fission in sexless beings. In the Aphides, ova are hatched 

 in spring, but ten or more generations are produced vi- 

 viparously and without sexual union throughout the sum- 

 mer. In autumn, however, the final brood are winged 

 males arid wingless females, from whose union ova are 

 produced in the ordinary manner. 



11. Transformation and metamorphosis relate to certain 

 changes or variations of development in the structure and 

 life history of an individual. Thus an insect is an egg or 

 ovum, a caterpillar or larva, a pupa or chrysalis, and an 

 imago or perfect insect, and these changes of condition 

 and structure constitute its development. Much difficulty 

 is caused by the phenomena of metamorphosis in assign- 

 ing the place of different species, transformations being 

 often mistaken for specific differences. It was formerly 



