now produced, from which again parthenogenetic
generations arise (K. llertwig).
t'ALSES OF Asexual Repkouuction. Taking all tlicse cases together, asexual reproduction or ' j)artlienogunesis in general is seen to be due to budding or cell-division in the egg. The asexual aphids bud out from the ovary. It was for a long time lield, says Hertwig, that the cells from which the cercariiP arose were not eggs, but 'in- ternal buds,' 'germinal granules.' The fact seems to be that normal rei)roduction with growth and parthenogenesis are but extremes of a single series. The asexual mode of reproduction is mo.st probably due to temperature and other changes in the conditions of life, as change of food, and, in parasitic animals, change of liost (Packard). The phenomenon takes place in the summer, and in almost every case ceases at the approach of cold weather.
The case of dimorphism of a thread-worm has bjcn incorrectly regarded as heterogony, but it is simply due to changes of temperature. Thus in l-.'vv temperature filaria-forms directly arise from t/ie young of Hhfilnhincmd (Aiiiiiiilhild) iiifrsti- «a//.v,- but in the suiimier heat indirectly forms a Rhabdilis form. 'asman was able, during tliri'c summers, to induce parthenogenesis in the V orkers of Foniiica. aanyuinea and their slaves [t'ormica pisca) by artificially warming the irests. Abundant food favors parthenogenesis, t'»lough in Phylloxera the stoppage of tlie food supply induces parthenogenesis. See Effects of Chiiiiyes of Temperature, under E'OLUTlOi.
Artificial T'arthenocexesis. Experiments by llcrbst. R. Hertwig. T. H. Morgan, and espe- eiallj' by Loeb, show that the unfertilized eggs of tile sea-urchin may be so stimulated by chemi- cal solutions as to undergo the earlier phases of development. Herbst experimented with potas- sium chloride and lithium chloride, but found that while the lar;Te developed, they were mon- strous and finally perished. Hertwig and also Morgan showed that if imfertilized eggs be treated by weak solutions of sodium chloride, magnesium chloride, or strychnine, they exhibit some of the prcjiaratory changes of yolk-division, and might actually divide, though without pro- ducing an embryo. Loeb finally succeeded in rearing large numbers of perfect larva> • from eggs which, without fertilization, are first treated •with a weak solution of magnesium chloride, and then transferred to normal sea-water. It thus appears that experiments cai-ried out under rigidly controlled conditions show that the egg, without union with a sperm-cell, is capable of complete development. In commenting on these discoveries E. B. Wilson (Internationnl Monthli/^ .July, 1900) remarks that even in normal fer- tilization we must regard the stimulus to devel- opment as being given by a specific substance or substances carried by the spermatozoon. The ex- periments lead us to suppose that the chemical sa,lts used "are individually poisonous to the egg, but are normally so balanced as to neu- tralize one another's injurious effects and main- tain the equilibrium of tlie egg. If this armed neutrality be disturbed, the egg responds, under- going degenerative changes, and dying if the change be too violent, passing through an abnor- mal development and giving rise to monstroiis embryos if the new conditions be less unfavorable, but under appropriate .stimulus being, as it were. released from bondage, and rendered free to its normal course of development."
In Plants. Among plants an embryo is fre- quently formed by a budding outgrowth from tissues outside of the egg. and to such a phenom- enon the term parthenogenesis has often been wrongly applied. The general term covering all cases of the appearance of embryos without fer- tilization, when that process is ordinarily re- quired, is apogamy (q.v.). partbenogenesis being that form of apogamy in which the embryo comes from an unfertilized egg. In tracing the origin of sex among plants, it seems evident thai the se.xual cells (gametes) are derived from such cells as the sexless swimming spores (zoospores) of the alga^. These spores have the power of producing new plants, so that wlicn sexual cells, without pairing in the process of fertilization, in- dependently produce new plants, it is simply the rcsum])tion of an ancestral power. The most primitive sexual cells show no distinction of sex, and in such cases parthenogenesis is very com- mon. In the higher development of sexuality, however, the pairing gametes are very milike, becoming the sperm and egg. and in such cases parthenogenesis is more rare, and occurs only in connection with the egg.
By the process of fertilization the egg is in- duced to divide, and this is the beginning of an endiryo. Certain eggs may be artificially in- duced to divide by other means than fertilization, and to produce embryos. Just what the stimulus is which induces the egg to divide is yet unde- termined, but it seems -probable that natural .]iartlienogenesis results from the same general conditions that obtain in artificial partheno- genesis.
As might be expected, partlienogenesis in plants is most common among the alga* and fun- gi, there being well-known cases, as the water molds (Saprolegnia) , in which the eggs are never fertilized, and parthenogenesis is the nor- mal method of embryo formation. Among the higher groups of plants, however, it is the exception, and among seed-plants (sperniato- phytes) probably the very rare exception. But three cases of real parthenogenesis among seed- ])lants have been determined, though doubtless there are others. These are in certain species of Antennaria (Composit.T) , of Alchemilla (Rosa- cea"), and of Thalictrum (Ranunculacea>) . Bibliography. Saks, "Zur Entwicklungsge- schichte der Mollusken und Zoiiphyten," in Wiegmann's Archiv fiir Naturfjeschirhle. i. (Ber- lin. 1837) ; Steenstrup, "On the Alternation of (fenerations." in Transactions of the lioyul So- ciety (London, 1S4.5) ; Owen, On Piirthenoycne- sis (ib., 1849) ; id.. Lectures on the Comparative Anatomy and Physioloyy of Invertelyrate Ani- mals (2d ed., 1855) : Huxley. "On the Agamic IV'pi'oduction and Jlorphology of Aphis," in Transactions of the Linnean Society of London, vol. xxii. (London, 18.58) ; Siebold, "On a True Parthenogenesis in Moths and Bees," in Trans- actions of the Koyal Society (ib., 1857) ; id., Beitriiye zur Parthenogenesis der Arthropoden (Leipzig. 1871); Leuckart, "Sur I'arrhenotokie et la parthe'nog^n&se des abeilles et des autres bymenoptfres qui vivent en societe." in Bulletin dc rAcadcmie des Sciences de Brtixelles,seiea ii., vol. iii. (Brussels, 1857) ; id., Zur Kenntniss des Gencrationswechsels und der Parthenogenesis
