130 GENERAL PRINCIPLES OF ZOOLOGY 



tional circumstance that reproduction is accomplished by animals which 

 have not completed their normal development; for example, the larvae of 

 certain flies reproduce before they have passed through the pupal stage. 

 Paedogenesis consequently is parthenogenesis in an immature organism. 



Parthenogenesis and Typical Amphigony. There is no absolute 

 distinction between parthenogenetic eggs and those needing fertilization. 

 On the other hand, their equivalency is fully shown in cases as the bees, 

 where the queen decides at the moment of oviposition whether the egg 

 shall receive a spermatozoan or not, this decision determining further 

 whether the egg shall develop into a female (fertilized) or a male (unfer- 

 tilized). Parthenogenesis is, therefore, not an asexual reproduction 

 which was antecedent to sexual reproduction, but rather one which must 

 have been derived from the sexual; it is a sexual reproduction in ivJiicJi a 

 degeneration of fertilization has taken place. It is, therefore, more in 

 accord with the natural relations to contrast reproduction by sex-cells 

 with vegetative or growth reproduction (division, budding) rather than 

 asexual with sexual reproduction. 



Sexual and Somatic Cells. The distinction of sexual cells from the 

 asexual reproductive bodies, the parts arising by division and budding, is 

 shown by their relations to the vital processes of animals. The cells of a 

 bud had a share in the vital processes of the animal before the beginning of 

 reproduction; they were functional or somatic cells. In the fresh-water polyp 

 (fig. 93), when a bud arises, the cellular material employed is that which was 

 previously related to the mother animal in exactly the same manner as the other 

 parts of the body wall. The sexual cells of an animal, on the contrary, are 

 excluded from the vital processes, remaining in a resting condition, and con- 

 serving their vital energies. Asexual reproduction is closely related to growth; 

 sexual reproduction is not even a special form of it, but a complete renewal of the 

 organism, a rejuvenescence of it. This explains the fact that asexual repro- 

 duction is most common in the lower animals (coelenterates, worms), but is 

 lacking from vertebrates, molluscs, and arthropods. The higher the organiza- 

 tion of the animal the more the energies of its cells must be employed to meet the 

 increasing demands upon their functional capacity, and so the more necessary 

 is sexual reproduction. It is farther noteworthy that fission and budding occur 

 most frequently in attached, sessile, or slightly moving animals (ccelenterates, 

 polyzoa, ascidians, oligochaates), an indication that the distribution of asexual 

 reproduction may be determined by the method of life. 



c. Combined Modes of Reproduction. 



Very often two modes of reproduction occur in the same species side by 

 side. Many corals and worms have the power of multiplying by division 

 or budding, and also of forming sex cells; other animals have no asexual 

 reproduction, but their eggs develop according to circumstances, either 

 parthenogenetically or after fertilization. The appearance of two kinds 

 of reproduction is very often governed by the fact that individuals with 



