130 GENERAL TRIXCirLES OF ZOOLOGY 



tional circumstance that reproduction is accomplished by animals which 

 have not completed their normal developmenl; for example, the larv;e of 

 certain ihes reproduce before thev have passed through the pupal stage. 

 P;i?dogenesis consequently is parthenogenesis in an immature organism. 



Parthenogenesis and Typical Amphigony. — There is no absolute 

 distinction betweeir parthenogenetic eggs and those needing fertilization. 

 On the other hand, their ecpiivalency is fulh' shown in cases as the bees, 

 where the queen decides at the moment of o\iposilion whether the egg 

 shall receive a spermatozoan or not, this decision deterxnining further 

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

 tilized), rarlhenogenesis is, therefore, not an asexual reprotluction 

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

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

 dcgciicralio)! of Jcrtilizalioii has lalccii place. It is, therefore, more in 

 accord with tlie natural relations to contrast reproduction bv sex-cells 

 with vegetative or growth reproduction (division, budihng) 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 SiUiiatic 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 

 e.xcluded from the vital processes, remaining in a resting condition, ancl con- 

 ser\ing their vital energies, .\sexual I'cproduction is closelv related to growth; 

 se.xual reproduction is not even a special form of it, but a com]ilete renewal of the 

 organism, a reju\'enescence of it. This explains the fact that asexual repro- 

 duction is most common in the lower animals (cadenterates, 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 enijiloyed to meet the 

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

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

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

 polyzoa, ascidians, olis^oclKetes), an indication that the distribution of asexual 

 reproduction may be determined by the method of life. 



c. Combined Modes of Reproduclion. 



\^cry often two modes of reproduclion occur in the same species side by 

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

 or budding, and also of fiu-ming sex cells; other animals have no asexual 

 reproduction, Imt 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 indi\-iduals with 



