Vickipedia

aphis, which in the perfect state possess wings, a large proportion of the individuals never acquire these organs, but remain in the condition of larvae. These without any sexual union (none of them, indeed, being males) bring forth during the summer living young ones resembling themselves; and these young ones repeat the process, till ten or eleven successive broods are thus produced; the last progeny, towards the end of the summer, being winged males and female”>

GENERATIONS,alteration of, a phrase devised by Steenstrup, a Danish naturalist, about the year 1840, to signify ‘the remarkable and till now inexplicable natural phenomenon of an animal producing an offspring, which at no time resembles its parent, but which, on the other hand, itself brings forth a progeny which returns in its form and nature to the parent animal, so that the maternal animal does not meet with its resemblance in its own brood, but in its descendants in the second, third, or fourth degree of generation; this always taking place in the different animals which exhibit the phenomenon in a determinate generation, or with the intervention of a determinate number of generations.’

The phenomenon has been observed in many of the Jiydrozoa, in various entozoa, in annelids, in molluscoids (salpce), and in insects (aphides); and its nature will be best understood by our giving one or two illustrations.

We commence with the development of the medusae or jelly fishes, which belong to the class Tcydrozoa. The medusa discharges living young, which, after having burst the covering of the egg, swim about freely for some time in the body of the mother. When first discharged or born, they have no resemblance whatever to the perfect medusae, but are little cylindrical bodies (fig. 1, a), covered with cilia, moving with considerable rapidity, and resembling infusoria. After moving freely in the water for some days, each little animal fixes itself to some object by one extremity (e), while at the opposite extremity a depression is gradually formed, the four corners (b, /) becoming elongated,

a n d gradually transformed into tentacles (c). These tentacles increase in number till the whole of the upper margin is covered with them (g). Transverse wrinkles are then seen on the body at regular intervals, appearing first above, and then extending downwards. As these wrinkles grow deeper, the edge of each segment presents a toothed appearance, so that the organism resembles an ar-tichoke or pine-cone, surmounted by a tuft of tentacles (Ti). The segments gradually become more separated, until they are united by only a very slender axis, when they resemble a pile of shallow cups placed within each other (i). At length the upper segment disengages itself, and then the others in succession. Each segment (d) continues to develop itself until it becomes a complete medusa (&); while the basis or stalk remains, and produces a new colony. Here, then, we have the egg of the medusa gradually developed into the polypoid organism (A), to which the term strofiila (from strobilos, a pine-cone) has been given. This polype, by gemmation and fission, yields medusas with reproductive organs.

The phenomenon of alternation of generations in the Cestoid Worms (q. v.),and in certain Trematoid Worms (see fluke),has already been noticed, and will be further discussed in the articletapeworms.The fission of certain annelids (Syllis and Myri-anida), (seereproduction),presents an example, although at first sight a less obvious one, of alternation of generations, the non-sexual parent worm yielding by fissure progeny containing spermatozoa and ova, from which again a non-sexual generation is produced.

The Salpa (mollusca or molluscoids belonging to the family Tuni-cata) are usually regarded as affording a good illustration of the phenomenon under consideration. It was in these animals that it was originally noticed by Chamisso, who accompanied Kotzebue in his voyage round the world (1815 — 1818).

TheSalpse (from twenty to forty in number) are united together by special organs of attachment, so as to form long chains, which float in the sea, the mouth(to),however, being free in each. The individuals thus joined in chains (fig. 2, A) produce eggs; one egg

being generally developed in the body of each animal. This egg, when hatched, produces a little mollusc (fig. 2, B), which remains solitary, differs in many respects from the parent, does not produce an egg, but propagates by a kind of internal gemmation, which gives rise to chains already seen within the body

of the parent, which finally bursts and liberates them. These chains, again, bring forth solitary individuals.

The chief instance in which this phenomenon occurs in animals so highly organized as insects is in the Aphides, or Plant-lice. In many species of the genus aphis, which in the perfect state possess wings, a large proportion of the individuals never acquire these organs, but remain in the condition of larvae. These without any sexual union (none of them, indeed, being males) bring forth during the summer living young ones resembling themselves; and these young ones repeat the process, till ten or eleven successive broods are thus produced; the last progeny, towards the end of the summer, being winged males and females, which produce fruitful eggs that retain their vitality during the winter, and give birth to a new generation in the spring, long after their parents have perished. Another instance is the Gynips or Gallfly. Hof meister discovered an alternation of generations in plants. For generation other than by impregnation, Owen invented the termparthenogenesis.

Cycle of Generationsis a truer term than alternation of generations; as there may be four changes between one several generation and the next for several series. Many authorities object to the term ‘ alternation of generations.’ The detached portions of the stock originating in a single generative act are termed Zb’oids by these writers, whilst by the term animal or entire animal (the equivalent of Zoori) they understand in the lower tribes, as in the higher, the collective product of a single generative act. Here they include under the title of one generation all that intervenes between one generative act and the next. ‘ If,’ says Dr. Carpenter, ‘ the phenomena be viewed under this aspect, it will be obvious that the so-called ” alternation of generations” has no real existence; since in every case the whole series of forms which is evolved by continuous development from one generative act repeats itself precisely in the products of the next generative act. The alter, nation, which is very frequently presented in the forms of the lower animals, is between the products of the generative act and the products of gemmation, and the most important difference between them usually consists in this—that the former do not con. tain the generative apparatus which is evolved in the latter alone.

The generating zooid may be merely a segment cast off from the body at large, as in the case of the Tape-worms (q. v.), or it may contain a combination of generative and locomotive organs, as in the self-dividing Annelide. It may possess, however, not merely locomotive organs, but a complete nutritive apparatus of its own, which is the case in all those instances in which the zSoid is cast off in an early stage of its development, and has to attain an increased size, and frequently also to evolve the generative organs, subsequently to its detachment; of this we have examples in the Medusas budded off from Hydroid Polypes, and in the aggregate Salpce-’ For fuller details see Balfour’s Embryoloqy (1880-81).